I am having the exact same problem with my '91 F250... I believe my problem started one day when I was turning on my headlights via the switch and I heard a tiny "pop" noise followed by a tiny metallic sound as if something like a washer had popped off and fell off the backside of the switch. My headlights still work but no interior/dash lights now. My guess is that I/we need a new switch. Good luck!