Go digging behind the headlight switch on the left hand side of the dash. On the plug that plugs into the back of that switch has a black wire that, if you follow it, it will joint with another black wire within about 2 inches of the switch. (in the 99 model year anyway, do not know about other years)
These 2 black wires are what I tapped into to get power to the LED's that I added to light up the floor. They come on with the interior lights.