I'm not sure about a Gen 3, but I did this to my Gen 2. Like yours my car didn't have the power wires either. However, the power wire for the mirrors just unplugged from the power for the dome light. So I got the mirrors and the wire from the junkyard and just plugged it into the dome light power and it worked fine. There wasn't any soldering or anything, just a plug. But this was from a Gen 2, and actually I think the mirrors and power wire were from a Gen 1.
But take off the visors and dome light and pull down the ceiling a little bit and see if there is a little plug somewhere, or just wire it into the dome light power if there isn't.