Here's a small how-to for others who might also want to add an autodimming mirror. I'll add pictures this evening.
1 - Tap into a switched +12v power and ground source using an add-a-fuse adapter. I bought a pack of 10 from Amazon for about $8.
Due to the added height of the add-a-fuse, I installed it in the underhood fuse box. I used the fuse for the vapor canister (mini fuse 13), as it was the only switched power source I found. All others were hot +12v constant.
As an alternative, you could use some kind of vampire tap and get switched power from inside the cab.
2 - Add a ground connection. I used a bolt on the driver's fender right next to the fuse box to keep things together. Use a ring or spade connector.
3 - Run two wires (preferably red / black) through the firewall. You can poke them through the grommet that's common to the hood release cable. I didn't have to cut anything, just poke it through enough to get a hand on it from the inside.
4 - Run the wires under the dash, using cable ties to keep it out of the way. Run it along the hood release cable, and across and up the side of the dash, until you can see it near the interior fuse box.
5 - Remove the a-pillar trim and run the wires up the a-pillar.
6 - Run the wires across the headliner to the center of the windshield. Tuck the wires under the headliner. I didn't need to loosen the headliner, just gently pull it down. Keep in mind the headliner has fiberglass in it (or something that feels like it and will make your skin itch), so wear some gloves.
7 - Install the autodimming mirror. I used one from a 02-05 Explorer without a compass or thermometer. It makes the install easier as there are only two wires. Mine was $28 shipped, and it looks like there are plenty at low price points.
8 - Splice the power wire and ground wire to the mirror harness. Black (pin 2) is ground. Pin 1 is switched power. Pin 3 is the reverse disable which I did not want, so I removed the pin. I used heatshrink bullet connectors to make the connection.
9 - Tidy things up with some wire loom and you are done.
