The trailer wiring kit (available either bundled with the hitch, or separately) includes the trailer wiring module (white box), the wiring harness, and the plug receptacle that mounts next to the hitch ball.

When I looked at pricing for the wiring kit and the OE hitch, it was significantly cheaper to buy both together than separately (in North America). There were aftermarket suppliers of the OE wiring kit that were cheaper than the BMW wiring kit price, but not cheaper than the BMW bundle price.

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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