Absolutely need the factory wiring harness. It does 2 things for you- 1) when the trailer is plugged in it disables the rear park sensors, and 2) it works without having to cut up the factory wiring like all the aftermarket pieces.

Also, you will need the dealer to initialize the module to the car too. Many have complained about their blinkers flashing fast, or that the trailer parking lights will flash by themselves every few minutes, even when the X5 is turned off. Initializing cured both of those problems for me.

It does NOT provide a place for wiring in an electric brake controller, you have to do that yourself.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R