The dealer sells the OEM wiring harness. It includes a plug-in harness, and a trailer light control module. It is specific to the X5. The reason to use it is that the BMW lights are multiplexed, and have a monitoring circuit that measures resistance on each wiring circuit. That is the monitor that tells you if you have a bulb out.

It is possible to simply use T-splices from the rear lights. Some have done it with no reported problems. Others have reported that the vehicle threw fault codes. Entirely up to you. I used the OEM harness, not just for the above issue, but because it enables the trailer mode on the PDC (disables rear PDC sensors so that it doesn't beep all the time due to the trailer).

The OEM harness and hitch are more expensive if purchased separately.

Good luck.

PS: Welcome to X5 World. In future, it is better to start a new thread for a question, instead of extending an old thread.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue