dome lights

I have a 93 suburban. I recently lost the dome lights up on the roof, ( middle and back). The lights at your feet still come on when the doors open. There is power at them. If I ground them they will come on but then they don't turn off with the doors. It must be a ground issue. I've heard that there can be lots of odd things happen if you loose a ground somewhere. Can anybody tell me where to start to find this ground?

HEY! I found the problem. Someone told me to look at the wires under the plastic piece on the floor under the drivers door. There is two bundles of wires that run to the back. I looked at them b4 but didn't find anything. Nothing broken. I looked closer this time and found a swollen spot on the white wire. I took a chance and cut it and it was completely rotten inside. (never seen that b4). I spliced in a new piece and it all works. Hope this will help someone else.