1989 GMC C1500

Check the brake light circuit boards (the part that the bulbs plug into). Remove the brake light housing, unplug the brake light connector. Look for burned connectors and/or wires. Also check the circuit borad plug-in (female connection). If there are burned connectors replace the circuit board (sometimes you will have to get a new wire connector also---it's the plug-in with about 12 inches of wire attached --found at the dealership).