Thread: DIY: B6 A4/S4 - Trailer hitch installation

After numerous requests to write up how I installed the hitch on my B6 avant, I have finally gotten around to doing it (at least the first half of it, anyway):


The B6 A4/S4 hitch install is not difficult, but it is a bit time consuming if you want to be OCD like me and demand that it be perfectly aligned with extra rustproofing and maximum beer drinking time while working.

That being said, here is the disclaimer: I am NOT a mechanic, trusting my instructions for something that is even as harmless as installing a solid metal bar onto a non-running car could cost you your life, limbs, eyes, dignity, manhood (or womanhood), and can definitely cause injury to pets and children. This install requires the use of several complicated techniques such as using a BFH, pipes, jacks, spraypaint, wrenches, a rubber BFH, a power drill, sealant, alcohol, and more spraypaint. The wiring of the trailer lighting system is not complex but it does require the "audi touch" to know the difference between when you are about to pop off an interior panel in once graceful motion and when you are about to break the damn think in half, gouge your hand and/or rip off a fingernail, and devolve into symphony of cuss words.

That being said: Attempt at your own risk, don't blame me if you mess it up!


Ok the hitch used for this install is the Curt Manufucturing 2002-2005 A4/S4 bolt on hitch, Model number 11164. $145 from etrailer.com. This hitch comes with the following:

-1 Hitch
-4 bolts/nuts/BIG fender washers/lock washers to attach through the tire well
-1 bolt/nut/washer/steel plate to attach through the tow loop
-1 ball mount, custom made for the A4
-1 hitch pin/clip

In addition to the parts listed above, you will need:

-Spraypaint of some kind, I used flat black rustoleum and Plasti-Dip
-2 addition washers big enough to fit the tow loop bolt (3/8" maybe?)
-a silicon sealant of some sort.
-jack, jackstands, etc
-rubber hammer
-metal hammer
-some various SAE wrenches, as Curt provides SAE mounting hardware
-power drill and some drill bits. 1/8", 1/4", 3/8"
-a center punch to mark the drilling location
-a little patience and probably a couple beers
-help is not really necessary, but as Morgan would say, it's nice to have someone sitting around to not help and to poke fun at you while you slave away.

The total cost for the hitch, wiring, stainless locking pin, Masterlock trailer lock, ball, and shipping (free) was $237. Not too bad!

Ok, now for the writeup:

The hitch will look like this out of the box:


Jack up the rear of the car (I used a lift, b/c I happen to have access to one), and mock up the hitch above the loop. You will need to loosen the two plastic lower bumper valance supports. One of mine was broken, so I only had to loosen one and saved myself at least a minute in labor.

Initial mock up:


Go to your jar of miscellaneous nuts/bolts/washers and find the two that will fit on your tow loop bolt:


And put it all together like so:


Now tighten it down pretty snug (not super snug) and see how she sits. On my car, the drivers side was a little lower than the passenger side, and the hitch receiver itself was pointing downwards a bit. Now its time to journey into the world of shade tree solutions 101:

First, we need to get the receiver nice and level, so grab some angle iron (or whatever you have aroun, a small diameter (<1.25") pipe would work), and set up like so:


Now Aristotle himself would be proud, you have made yourself a LEVER! Like I said, my receiver was pointing down a bit, so just lift up a bit! Problem solved? Not quite, as the drivers side was still hanging a bit low. Enter the pipe and jack: Get a pipe, 2x4, cinder block, etc and apply pressure to the drivers side edge of the hitch to get it level, as shown here:


Now that I had it level side to side, and could press the receiver up and level with the angle iron, I was almost set to mark my centers and drill. Except I was working alone, and had nobody to hold the angle iron up to get the whole hitch level. Enter jack # 2, plus the handle from jack # 1, plus a shaky cell phone camera, and you get:


Thats right, you level it side to side first, then cannibalize the first jack's handle b/c you have no more pipes or angle irons, then use that to press on the angle iron using jack # 2. Genious, I know!

Now that its all level and in place, you are ready to mark the drill centers with a center punch and the BFH. You do this from the outside of the car, and must start drilling from the outside in with the 1/8" bit. disassemble the ingenious jack on jack leveling Rube-Goldberg (after lamenting its genious one last time), and remove the hitch from the car completely. Once the holes are piloted, then you open the trunk and drill them bigger from the inside out. Get it? No pics for help on this, if you cant figure out how to drill, you are truly mechanically retarded.

I suggest that before you punch the marks, take a good long look to make sure the hitch is where you want it, above the tow loop (I have seen it installed below it by U-Haul in a post here on AZ!). This is the point of NO RETURN.

Ok, now that its all drilled and ready to go you should have four little holes that you purposely put directly though the unit body of your precious Audi. Yikes! Better hit them with some spraypaint (inside and outside, make sure t wipe off dirt/grime before painting) to prevent corrosion, and it'll look like this:



As I was waiting for the rustoleum to dry, I decided to peel off the Curt stickets, scuff up the powdercoat and repaint it with Plasti-Dip to get a matte finish. I also painted the steel plate that used in the tow loop. I applied several coats and the finished this install the following weekend.



When you are ready to put in on the car for good, start by again placing it over the tow loop and snugging it down. You'll want (IMO) to use some silicone to seal the holes, etc. I started by applying a ring of sealant to the outside of the drilled holes from beneath the car, then mocked up the newly painted hitch, and started placing the bolts through from the inside. It looked like this:


You can easily get one side all together, but of course it wont line up perfectly on the other side and you'll have this:


Apply the angle iron for leverage and slide the fouth bolt into place:


And it'll look like this:


And like this from below, after you apply some more sealant:


Inside:


All tightened up, pre-final spraypaint:


Spraypaint over the bolts/sealant etc once its dry, and do what you can to make it as corrosion resistant as possible.

And the final product:






No, its not the prettiest thing, it is probably somewhere behind an Oetty front bumper in terms of adding aesthetics to the car, but it does allow you to do this!



Im going to do the writeup for the wiring installation sometime soon, once I find the pics. It does involve removing a lot of the trim in the rear (at least in an avant) and tapping some wires, as well as running a power cable to the battery through the firewall.



PM with questions. Given the amount of pics and explaination for a really simple install, I expect that install will be a breeze. Once you're done, go crack that eighth cold beer of the afternoon and marvel at your beautiful (in the eye of the slightly intoxicated beholder, of course) new hitch. Then sober up and go tow something.

What's our towing capacity? 1 Jetski/motorcycle or less?

All kidding aside, nice writeup.

The hitch is rated to 2000 pounds, but the most I have pulled is around 1,100 pounds on the trailer (two motorcycles) with two people and a car completely full of track gear. So thats about 400 pounds of human, ~200 pounds of gear, and 1,100 pounds in tow. It still drove very, very nicely. With one bike on the trailer you cant even tell its there.

I have seen pics of people towing pretty large boats though, certainly more than 1,000#.

BADASS... Im gonna do this just to lay sparks

Am I the only one that doesnt like seeing trailer hitches on cars. I guess on the Avant it isnt so bad but on a sedan :(

you can just take that extension piece off and you would never know

i would do this just so the assholes in ny would stay away from my damn bumper.

Are there hitch models that bolt into the frame directly? I would be scared to put more than 1,000 pounds on the ball.... Someone will find themselves loosing their spare tire well. That metal you drilled through looked fairly thin!

I've been searching high and low on hitches too as I need a hitch to tow my 5x8 trailer with race kart in it plus tools/spares Unfortunately in the NA market, only the Class I exists. The Euro hitches, ie Westfalia and Bosal, bolt directly to the frame but are only available in Europe. I think they are rated at 120kg on the hitch and 2,000kg trailer weight. They can be had but mucho $$$. The only downside to the EU hitches is that they don't come with safety chain hookup loops which are required by law in most states. Not sure how you'd go around that other than fabbing the hookup loops. I don't think the tow loop will suffice for that purpose though.

My buddy tows a similar trailer as mine with a 1.8T Avant equipped with the same hitch as lookaught and he said it's working great for him. He's been doing it for years and travels quite lengthy distances. First thing he told me about the hitch and A4, "Don't let anyone tell you that you cannot tow with these little puppies."

The spare tire well is actually surprisingly strong, the tow loop to get the car onto a flatbed is welded only to the bottom of it and can clearly support 3600+ pounds of the car. The metal itself was actually pretty hard to drill through, quite sturdy.

The B7 is a bolt on hitch, I dont know why they don't make one for the B6, though I didnt really investigate all that much as this was a pretty inexpensive option. I would have preferred if they had made the hitch receiver verticle and had a U shaped ballmount to hide the hitch (a guy with an A3 has this setup) but Curt decided to make it a standard horizontal design.


Also, to be honest I dont care for the look of the hitch either, but when you have one car and two motorcycles... you need a hitch. It sure beats owning a pickup or shitty SUV. I can over 25mpg on the highway fully loaded with this car/trailer setup.

i've got a 4x7 enclosed trailor that i'd like to be able to use to tow my dirtbike and motorcycle if needed, but w/ the body kit that was on the car when i got it, i dont think a hitch would be accessible.. maybe if i cut a square in the rear bumper and kinda took the same approach as the front tow hook in the bumper or maybe the headlight pissers.. we'll see. good write up...

I didnt have to trim my bumper at all, but Praz said he had to notch his B7 S-line a tiny bit to make it fit. I would expect that the S4 would need to be notched.

To me, it's a car. I only have one so it doesnt matter if I have to cut the bumper or not. I needed a hitch because I need to tow my motorcycle.

Alright I've finally found the damn pics of the wiring and I'm going to finish this writeup.

There are a few steps to the wiring:

1) Remove the trunk/cargo area paneling on both sides.
2) Remove the trunk sill trim
3) Clip the wire piggyback connectors to the correct wires
4) Run the power wire to either battery, or use the cigarette lighter in the trunk area
5) Place the converter box out of the way
6) Put it all back together

Additional Steps:
1.5) Lager
2.5) IPA
3.5) Belgian
4.5) Dunkelweiß
5.5) Tequila shot
6.5) Fuck it, get my beer bong

Ok removing the interior paneling. THis is not easy, in fact, it is by far the hardest and most annoying part of the entire install. Audi does not use many screws, it uses clips. Clips that break, and clips that break fingers and elicit words of condemnation that you don't even realize that you know.

These steps are for an avant, a sedan is probably similar, but I really can't say for sure. It's been about nine months since I actually did this, so the memory is getting foggy, but basically:

-remove the four eye loops on the cargo floor, they use torx bolts.
-fold the seats down, you may or may not need to remove the seat posts. I did because I wanted to get the trim all the way off to see what the hell I was doing.
-there are two little screws right where the headliner meets the lower carpeted paneling on the inside of the D pillar. Remove these (one per side)
-refer to picture below. I have highlighted the clips in RED and the holes they are cemented into in green. They pull strait out. This is where the "Audi touch" comes into play. be careful, pull firmly but don't, don't, don't be a steriod freak and rip at the panel. Try to get your hands/fingers as close to the clips as possible, and pray to whatever god/animal/beer/whore it is that you pray to that you don't break them. Repeat on both sides. GOOD LUCK.



There are six wires for the Modulite:

-power
-ground
-R turn
-L turn
-running lights
-brake

You need to wire these (I dont remember which is which, the Modulite has them listed by color) to the car. For the avant, there are two harnesses just below the drivers side D pillar (circled in RED) where you need to find the correct wires. These are the wires I tapped and the corresponding colors... You may want to test the wires with a multimeter, that's how I confirmed them after finding them on the wiring diagram in the Bentley. Or you can trust me (misguided or not), it's your choice:

These are on the left (driver's side) of the car:
Power - to the battery, you dumbass.
Ground - to the nut that is circled in orange
L Turn - Green/Blue wire, circled below in green. Follow the pigtail that comes from the left side lamp housing and tap wherever, I chose about half way because it was out of the way.



The running lights and brake lights were wired to the other harness that leads to the avants hatch that is circled in red. I did this for a reason: The normal side lamps use a combination running light/brake light, not independent bulbs. Therefore, I decided to use separate wires that would only have constant voltage and not varying voltage depending on if the brake was on or off. Refer to the pic below

So for the running lights, I tapped the wire that powers the license plate lights:

Running Lights - Gray wire. It comes out of the black harness (as opposed to the brown harness)

Brake - Red/Black wire. This wires is also on the black harness and powers the third brake light in the hatch.

You can see the brown wire coming up from the Modulite attaching to the gray audi wire for the running lights, and the red wire coming from the Modulite is attached (can't see this in the pic) to the red/black audi third brake light wire.



That leaves only the right side turn signal. You need to run this modulite wire (it is extra long) to the right side of the car under the trunk sill. No pic for help with this. In the Avant, I need to remove the subwoofer for access to the right side lamp wiring, and tapped the Green/Black wire coming from the lamp pigtail

R Turn - Green/Black on right side of the car


Running the power wire can be tricky. I ran it along the left side of the cargo area, down under the rear seats, then tucked into the plastic trip along the footwells, then up through the ECU box (this is really easy if you open the ECU cover from inside the engine bay and look down, you can literally see the pedals from the ECU). Wire it to the battery using the inline fuse provided with the wiring kit. Alternatively, you can wire it to the cigarette lighter outlet in the trunk. I didnt do this because I wanted the Modulite on a separate fuse, and as I had already paid for the extra wiring, I'll be damned if I wasn't going to effing use it! It is not hard to get it to the battery. If you attach it to the cigarette outlet, I suggest that you solder it and dont crimp.

Ok, finally, you need to get the Modulite box and wiring out of the way. I used the little empty hole below the right tail lamp housing (see the purple circled area in the second pic with the colored circles above), and ran the trailer pigtail out into the spare tire well, where it lives full time next to my spare tire.

Like so:



Before you reinstall ANYTHING double check all connections and TEST THE SYSTEM on a trailer. The last thing that you want to do is re-install all the trim and find out you've effed something up and have to take it all out again.

Now carefully reinstall all of the paneling, bong another beer, do a little dance, pet a little dog, tuck all the wiring away and your DONE!

As a final note, when you use this wire and it's hanging out of your trunk, tape off your trunk with painters tape so that the wire doesnt rub the paint, as it will scratch the paint!!! Just guess how I know this?

Good luck and godspeed grasshoppers!

Just finished this project.
A few pertinent notables:

'03 Avant with full H&R coilovers/APR exhaust.

Marking and drilling:
There is very little room and maneuverability, underneath the car, by the hitch mounting points (APR exhaust-may take up more room under the car compared to stock(??).
Either way, it was tight. So, I used an 18 inch drill bit, instead of a punch, to mark the holes. The length of the bit let me access the holes in the hitch from the back of the car. It was either this or drop the exhaust!
Due to the tight quarters I had to work in, when I drilled the initial holes, I used a 1/4" right angle grinder with a 1/4" drill bit (I did not have a right angle drill). The high speed of the angle grinder will pretty much toast the bit....but it took no time at all to drill 4 holes. I then brought the holes up to size from the inside out.

Wiring:
Driver's side
The panel, as has been discussed, is a bit of pain. I took the drivers side panel off. This is necessary, as you will need the direct access to make your connections.
(A note on the wiring harness desciption above. I found that the black/red wire for the high-mount-brake-light came out of the BROWN harness).
Passenger side
I did not remove the passenger side panel!!
Right turn signal:
Remove the rear light assembly and pull the rubber(seal) plug out of the body. Feed the wire through (from the inside-out), to just forward of the rubber plug. Tap the right-turn-signal wire. Stuff the newly tapped pigtail back into the body and replace the rubber seal. Remount the tail-light assembly.
Power:
For power, tap the 'cigarette' lighter in the cargo area. Unclip the line for the outlet and tap into the 'hot' wire {I did fuse this line(obviously, you will need a bit of wire with the appropriate inline fuse)}. Then plug the clip back into the lighter socket. This way, there is juice to the trailer harness...and a fully functional lighter.
Both of these passenger-side wiring actions can be done without removing the panel.


Overall, if you take your time, and have patience, it is a decent project.
I would like to thank Lookaught for his time and excellent write-up!!!
His pictures and descriptions made this project go very smoothly.
Many, many thanks!

I will post pictures when I get my new trailer axle assembled and back on the trailer.

girl paws? never heard that before, as a rock climber my hands are usually pretty much the opposite

but thanks.

Hitch in action.

Werd!

SUVs FTL!

Killer write-up man, and not only that, It was a good read too. I read the whole thing while taking a shit with my laptop on my lap.. TMI?

Brown is always ground on an Audi. So red is the power. Check the fusebox?

Put your multimeter to the 10V setting and ground the black lead. Then turn the four way flashers on and go back and touch the red lead to each pin in the right taillamp harness (unplugged of course) until you start to see one bumping up the voltage as the lights blink.

Um, yes. lol!

I hope the deuce went like this and not like this