1986 Jeep Comanche electrical

Incorrect lamp operation is almost always a sign of a broken ground wire. You can prove this by testing for voltage on the round brass base of the brake light bulb with a test light. You'll see the test light is dim or flashes dimly.

The most common cause of a bad ground is after someone connected a trailer wiring harness and made the splices with Scotch-Lok connectors. Those don't seal out moisture. The splices should be soldered, then sealed with heat-shrink tubing with hot-melt glue inside. You can get that at any auto parts store.