Grand Cherokee mirror wiring?

I bought an auto dimmer mirror from a grand cherokee, its the style with a off-min-max switch on the front.

It has 3 black wires on the back, does anyone know what the 3 wires go to? Obviously ones gonna be a ground and power, but which? and the third wire?

the third wire gets wired to the reverse lights, so that when you shift into reverse the mirror will undim........not really necessary imo

i didn't hook it up and i have had absolutely no trouble.........

test the wires with a 12V battery to see which ones are the + and -

yeah reversing doesnt seem necessary. thanks for the info.