I refused to pay for that $200 circuit board that BMW recommends for towing lights. My hitch guys just hardwired the light harness (running lights, left/right brake/turn) to the cars wires which run to the appropriate lights.
We used a "converter" that takes the three wires of the car to the two wires of the trailer. Worked fine with single jetski trailer with reg lights. I bought a double jetski trailer with many more lights and I had issues. I switched to LED's which draw much less current and it works fine.
Hidden Hitch makes a whole line of these converters. The best way to go is to get the "power converter". It runs a line directly to battery for power. It only relies on vehicle light circuits for the "signal". Therefore shouldn't screw up BMW diagnostics.
http://www.hiddenhitch.com/
you can buy them online for $40 at
http://www.trailerhitches.com/electr...tion118176.cfm
I have an 01 X5, maybe they've done something else since, but I doubt it to prevent use of this product.
From attached install instructions:
Some vehicles combine stop and tail light
functions onto 1 circuit and then also have
an additional tail light function wire that is
independent of the stop
(such as the BMW X5).
These vehicles will require that you attach the
trailer light power module red "stop" wire to
the same combined circuit on the vehicle and
then attach the brown "tail light" circuit to the
independent circuit on the vehicle.