That's good to know there is a generic one - it's something I've thought of doing too.
Download a copy of the factory repair manual:
2007 Yaris Factory Repair Manual - Toyota Yaris Forums - Ultimate Yaris Enthusiast Site
It shows how to remove the headliner and everything else. Be careful not to bend the headliner or you'll leave a crease.
The easiest way to tap into the wiring is to get those clip on taps at any auto parts store. Get the right size for the gauge of wire there.