If you have the OE hitch installed, with the BMW wiring harness, there are some extra pins that came with it. This is for hooking up the brake controller. See page 13 of this
http://www.bimmerfest.com/forums/att...0&d=1330728890
You can use the the Tekonsha prodigy wireless controller here, if you do not want to run a cable from the right rear of the car where these connectors are to the front
Tekonsha Prodigy RF Brake Controller - Proportional - Trailer Mounted Tekonsha Brake Controller 90250
The brake contoller will need to be upfront with you.
As an FYI, I tow 6000 lbs, but my trailer has surge brakes. Its a tandem axle, with 4 wheel disk brakes, and I've never felt that it was under braked.