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S10 Pickup taillights & interior lights

I have a 1991 S10 pickup with 46,000 miles on it. When I bought it from my cousin, who inherited it when her granddad passed away, my cousin warned me the interior lights and taillights were hinky. When the headlights are on, the interior light stays on. The dash lights stop working, and the taillights stop working (brake lights are okay). I trust my mechanic. He has checked connections, wiring harness, etc. We just carry fuses around now. When the interior light stays on, we know it is time to replace a fuse. Neither heat, cold, rain, distance traveled, driving speed, nor driver are factors. Anyone else experienced such gremlins? Thanks!

You have a bad ground somewhere.

I used to have a '94 full-size Blazer that developed a similar issue. You would step on the brakes, and the instrument cluster and parking lamps lit up; it would also cause the left turn signal to blink super fast. The problem was an exposed wire around the trailer wiring harness that had rubbed against the body and was grounding out. Cutting and taping the wire solved the problem.

If you have a trailer hitch, carefully inspect the wires around it. Even if not, the problem could well be an exposed wire somewhere…finding it is the trick.

My guess would be an internal fault in the main lighting switch, or headlight switch if you want to call it that.

Thanks for all suggestions. My mechanic again went through what links he could without removing the dash, and I secured the trailer wiring over the weekend. No problems for three days, a record for this truck’s wiring system. Thanks for all posts.