I agree with jp1203. Though you may be able to grab power from the map lights if that is all that is missing. I'm sure the mirror would be easy to mount since they mount to the windshield, just remove the factory one at the head liner. In my opinion I think they are very bulky and get in the way of your vision out the front. But then I don't come across much traffic and rarely ever dim my mirror anyhow. The only thing that would make one beneficial to me is if it had the home link for the garage door opener.
Either way though let us know how it goes. It would be nice to know that you could retrofit one with just tapping into power, or what else is needed. Could be a very good upgrade for many people.
The 12V power was not available on all three of my nonsubie homelink/compass/auto-dim mirror installations. My kit supplied enough wire to travel down below the dash and search for some 12V power, preferably switched. IMO the hardest part of my installation was the removal of the original mirror.
I have upgraded 2 rear view mirrors on my 2 previous Foresters. The plug under the headliner accomodated both of them without any problems. If you just need plain power, you can run wires from your dome light...when you tap into them.