The trailer hitch receiver is a 2" square socket under the rear bumper, into which you install a tow hitch (held in with a pin, removable when not in use).

The trailer control module is a wiring interface that enables trailer lights, and related functions.

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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