I installed an auto-dimming/compass mirror in my 07 and have installed several of them. Most of what you will find are manufactured by Gentex and the pinouts can be found online.
You should be able to unhook the connector and check to see if there is voltage across pins 1 and 2 easily enough. Also, if pin 3 has power it will disable the auto-dimming function (reverse).
Here's the typical pinout...
PIN 1 POWER +12 volts
PIN 2 CHASSIS, GND
PIN 3 REVERSE SIGNAL
2002 Mercury Cougar - Sport V6/MTX - Vortech/HMS/Centerline/Koni
2007 Focus ZX3 D20/MTX - AVIC Navigation/Alpine Amp/Infinity Speakers/ OEM SVT Sub - Gunmetal CSVT E1 Wheels - FRPP Suspension - Hitch
1997 F150 - The toy hauler