Did you install the full harness package with the module that goes in the spare tire well? If you have no power to the module, there must be a break in the wiring somewhere. But if you have power to the module but no power to the trailer plug receptacle, I'd bet the module is bad. Mine was plug and play.

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