My little brother was playing around and ripping off my rear view mirror in my 2002 TL (the factory autodimming one)

When i was wiring it back, I noticed it had 3 wires, I'm curious as to what the 3rd wire is for? (white, grey, black wires)

 

I would assume it's positive, negative and sensor for the auto dim?

 

Sensor is already in the mirror. positive, negative and remote??

 

2 positives (one for power, the other for the reverse signal) and 1 negative.

When you're in reverse, the reverse line should have +12V. The mirror's auto-dim function is disabled when you have +12V on that line (i.e. when you're in reverse).