Brampton Builder from this site (works in the factory building challengers) told me that you need two parts. The auto dimming mirror itself, and the harness that goes from the mirror over to the drivers side A piller. Apparently, there is a plug at the bottom end of the A pillar that this harness plugs into. That portion of the wiring is supposedly part of all cars, but the harness from the mirror to the A pillar is what is missing from our cars that don't have auto dimming already.
I've found a mirror/harness a couple of times, but the junkyards want huge bucks for them.. more than I wanted to pay.
Mike