I don't know if there is any single wire that powers all the coutesy lights in our cars. However, one guy put theater lighting on his tour; he did so by changing pretty much all his interior courtesy lights to LEDs then he wired in caps of smaller capacitence to them. I think this way would be better, because the LEDs draw less power, hence lower capacitence needed for the cap, which probably means lower price. But ya, i dunno where you could tap a sigle wire that actually powers all the lights rather than triggers them.
