Kalmback put put a book on DCC projects, which disscusses wiring lights into coaches. If you change the decoder, tjat should prove to be a good basis. probably this one
If the car is a Walthers, they can help with their info in the lighting kits. most of the light kits I've seen sit on a cieling attached to the car body. See some Walthers, and Spectrum Doodlebugs. If you have any, try opening up a Spectrum car. There are wipers there.
There's an article on the Wesport Terminal site that briefly mentions power pickup. A similar trick can be achieved with a Kadee centering spring.
-Morgan