If you have the BMW hitch with the trailer wiring socket, you can plug something in to the socket to tell it that there is a trailer there. That disables the rear PDC. No luck if you don't have the BMW trailer wiring harness.

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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