If it's original wiring, check for past splices into the harness under the dash and at each of the tail lights. Black electrical tape "junctions" if any exist should be checked for faults. If you have a multimeter, make sure live side of any of your light circuits read high impedance to a known good ground with battery disconnected (always do impedance tests on de energized circuits). Check to make sure that all light socket bodies read zero ohms to ground. Check wiring thoroughly for insulation breakdown. Make sure that any previously joined wires are either soldered or crimped and properly isolated. All the Best, M