I've been having the same problem with my 95 Mystique. So far I've traced it to the switch in the door frame. I find if I push it in a couple of times and wiggle it around, that the lights come on and the keys/lights-on/door open chime work again.
So since it looks like the switch is either dying or is just dirty (probably the latter from it's location), I'm going to try some electrical lube of some type. Possably silicon spray to clean it up.
