Marc;
One of the first things that I installed were courtesy lights under the dash.
You can not buy the lights from GM because they are part of the wiring harness and are not sold as a separate item. I got my from a salvage yard from a 1994 and a 1998 Sonama pickup truck.
The power comes from attaching to the supply wire that feeds the mirror lights. This way they function in unison. The connection is best made behind the kick panel in front of the passenger side door. I think the wire is grey with a black strip.( you better check the wiring diagram or you can use a tester).
Lights install using existing screws under the dash.
Makes a big difference on the interior look.