I thought about going this route for a long time. With the way Toyota wires their vehicles, I was worried that the harness for the homelink features/buttons would be different in my trail that did not originally come with the option. If memory serves, the pigtail for the overhead console may be different between the versions.

I ended up buying a frameless mirror and running a power wire to an add-a-fuse that only works when the key is in the "on" position. This keeps the buttons from being active when the car is parked. My 4R didn't have the auto-dim mirror originally, so I was able to add two features I didn't have with the mirror. If yours already has the auto-dim mirror you may not have to run the power wire all the way to the fuse box.