interior lights

I don't have any insturment lights and was wondering if anybody has a good wiring diagram for a '96 Metro. The way its looking I have lost my ground for the lighting circut. If I ground my dimmer wire off the back of my aftermarket radio I get dash lights, just not controlled through the dimmer rheostat. Is the dimmer rheostat where the ground for the insturment lights is picked up? How does this rheostat work? Is it controlling the ground or is it using a voltage drop across the rheostat to get its ground? I need some info. this is driving me crazy. Thanks