Solved: Aftermarket trailer wiring - shop won't do brake input?

[Attacking Mid]

Yes, it also affects your stability control. Good call to get the BMW harness and module. This is exactly how I did mine. I could not find an OEM BMW hitch (my first choice), so I settled for the Draw-Tite hitch, but with the BMW harness and module. I use an aftermarket Tekonsha brake controller. I frequently pull at/near the vehicle limits and it all works like a champ.

AM.

[RhythmMachine]

Closing the loop on this - I haven't towed with it yet, but purchased a Curt hitch, OEM wiring harness, and Tekonsha 90920 brake controller.

Tekonsha sells BMW wiring kits, but for $60 I figured I would try soldering my own connections first. So far, so good - the lights work as expected and the controller recognizes when the trailer is connected. I chose this Tekonsha model because it has a Bluetooth app as well (if Bluetooth disconnects it just uses it's current settings, so it's low-risk and my trailer is small). For now, rather than trying to run wiring all the way from the back of the car to the dash, I've just mounted the controller in the rear compartment near the fuse box. I can do initial settings from there, and then use the app as needed for on-the-fly adjustments, which have been very rare in the several years of towing we've had with this trailer with another vehicle.

Installing the hitch is not easy, due to the tight fit and lots of old, plastic bits and clips used to keep it in place. The OEM wiring harness was involved, but relatively straight-forward, apart from getting the grey 16-pin connector through the port from the rear compartment to the bumper. Eventually took the connector apart (in 2 pieces) and was able to get it through that way. I also misjudged how much extra cable I'd need to get the pigtail jack mounted on the hitch, and so had to try to cut zip ties and feed more through after the bumper was fully reinstalled. Wouldn't advise that.

For anyone who can do those first parts, wiring in the brake controller will be easy, though I'd expect more challenge if you're running wiring all the way through the interior of the car. For reference, here's how to match up the wires from a Tekonsha controller (first value) to the OEM wiring harness (second value):


Black - Battery+ = Red
Red - Brake switch = Yellow/black
White - Ground = Brown
Blue - Brake feed = Yellow/green