If you have a sunroof, which I see you do FireEngineer, you can splice into the sunroof switch power up in behind the switch. once you are done you just push the switch housing back up into the ceiling and the wires are behind it and not going anywhere. The link I posted is for installing a mirror in a RAV4, but it should be the same procedure when tapping into the sunroof power on a 4R, and the link has pictures. I did it in my RAV and it took @ 1 hour I think, and I was taking my time. I used vampire clips to tap into the power.
Thees instructions should work for auto dimming mirrors with or without a compass. If you get a mirror with outside temp then it will be more complicated to route the temp sensor, but you should still be able to use the sunroof power.
http://sumbiz03.home.comcast.net/~sumbiz03/ECMirror.pdf