I finally found the proper wire to the center brake light, hooked that up, and the thing now works! Earlier, they'd said all of the lights would work off the CANbus, but that isn't true on a BMW (but is on others). BMW has some weird, patented technique to control the lights, and a generic box can't decode it without patent issues, so that is the reason the brakes didn't come on and you must connect it to the center brake light. My guess is, the center brake light is the only brake light that actually gets 12vdc directly, the ones at the rear get some multiplexed signal that controls that cluster which is why they can make four different lighting functions work with only 3-wires and a ground lead (turn, brake, fog, running).
The OEM and the Right-Connections harness provide the following functions:
- enable trailer stability logic
- disable the rear PDC when connected
- adds a green centerline on the rearview camera screen (it's slightly offset to the left so you can still see the ball). At least, I don't remember that center line being there, only the outer ones previously.
- on the rear view camera, when connected, it shows a symbol for a trailer and does not display the proximity boxes when the PDC is active.
This harness provides a 20A, relay controlled caravan charging circuit, and a separate, always on frig circuit activated (I think) when it detects a trailer attached. The relay circuit only comes on when the vehicle is running.
So, there are four splices that need to be made, plus some connections to ground and to the battery. You connect to the right turn signal, the CANbus, and the center brake light. You must remove a lot of the trim in the rear - essentially all of the floor pieces and all along the right side to gain access to run the harness and make the connections.