For any of the mirrors without map lights, there is only one power wire. This same power wire supplies power to the self dimming, compass and temp features. Then there's a ground wire, the reverse feature wire, and if it has temp, another wire for the temp sensor plug in. Not sure what the 5th would be for.
And it's impossible not to tell if it's an OnStar. The OnStar buttons would be lined up beneath the mirror like so...
Otherwise, I presume you have a 7-pin harness on a mirror that looks like this?
