Preston-
I really don't know much about the Toyota systems as I'm a Ford guy with a Toyota, but I've put junkyard auto-dimming mirrors into a lot of my past cars. There are usually 3 wires on the Ford mirrors, power, ground, and an override for when the vehicle is put into reverse to un-dim.
I never worry about the reverse wire, I only put the power and ground wires. The power is hooked to my rearview mirror power wire, and I use a chassis ground, usually just a small self-tapping screw into the roof above the headliner. Works great.
Not sure if that helps a whole lot, and it would be totally different if you're running a compass, but that's how I added mine...
HERE is a writeup I did with my Explorer.