I have a 1999 Yukon that has ceiling lights that sometimes work and sometimes not. When they work they are dim. I would like to know where there are inline connectors in the power wire so that I can clean the connections or replace them. Please provide a photo or drawing of the connectors in the power line going to the interior lights and a drawing of the power line so that I can find and recognize them (it). Please also show me where the ground connection is made for the ceiling light circuit, considering all the rust on the frame, maybe that is causing the problem.

I recently purchased this vehicle and have found that everything on and in the frame have a lot of rust even though the body has nearly no rust. I suspect that there is a power line somewhere in or near the frame that has a connector that is causing the problem. The main ground connection to the frame was corroded off. I replaced this ground connection.