It could be several things, the dimmer switch is where i would start since it is a 1989. then the door jam switches, also make sure the lights them selves don't have the on, on with door, or off switch- make they are switched to door if you have them. as far as the ground, you will need to find the ground for the dimmer and check to see if all the door switches are connected/not grounding out from broken or damaged wires. start with 1 door at a time, depress the button if there is one ( i do not know when they switched to a non-mechanical/magnetic switch), is it solid moving and contacting the door (put a little chalk, some marker, or something that will transfer from the button to the door) you will need to make sure it presses in all the way especially if its those L shaped triggers, sometimes they have a little material added to them and it comes off. take it 1 step at a time and eventually you can find it. oh and also, there may be a relay involved, if the door triggers trigger a relay, it may be bad.