* Wiring Harness Issues: This is the most likely culprit. A break, short, or corrosion in the wiring harness leading to the taillights is common, especially in older vehicles. The harness could be damaged near the rear of the vehicle where it's subjected to flexing and movement. Inspect the wiring carefully, looking for broken wires, frayed insulation, or corrosion, especially around the connectors at the taillights and body.
* Grounding Problem: A poor ground connection can prevent the taillights from working. Check the ground wire(s) connected to the body of the vehicle near the taillight assembly. They might be corroded or loose. Clean the connection and ensure a good, solid ground.
* Tail Light Switch (Brake Light Switch): While the brake lights aren't working, it's possible the switch is failing intermittently or only failing when the brake pedal is pressed. A faulty switch could affect the tail lights too, depending on the wiring configuration. This is less likely given the tail lights are already out, but it's worth investigating.
* Multi-Function Switch (Turn Signal/Hazard Switch): Though your blinkers work, a problem in the multi-function switch *could* be affecting the tail light circuit, although this is less likely.
* Body Control Module (BCM) or related components (less likely): In some vehicles, a BCM controls lighting functions. However, it's less common for a BCM failure to cause only the tail lights and license plate light to fail while leaving the blinkers functional in a vehicle as old as a 1991 Explorer.
Troubleshooting Steps:
1. Thorough Visual Inspection: Carefully examine the wiring harness from the taillight assembly to the fuse box. Look for any signs of damage. Pay close attention to areas where the wires might rub against metal parts.
2. Test the Wiring: Using a multimeter, test the continuity of the wires from the fuse box to the taillights. Make sure you have power at the correct wires with the lights switched on.
3. Check Ground Connections: Clean and tighten all ground connections at the taillights and chassis.
4. Inspect the Tail Light Switch: Test the brake light switch with a multimeter to ensure it's working correctly. You should get continuity when the brake pedal is pressed.
5. Check the Bulbs (again): Double-check that you're testing the correct bulbs (sometimes they have similar-looking but different wattage bulbs). Also, make sure the sockets are clean and make good contact.
6. Consult a Wiring Diagram: A wiring diagram specific to your 1991 Ford Explorer will be invaluable. This will show you the path of the wiring and allow you to systematically trace the circuit. You can find these online or in a repair manual.
If you're uncomfortable working with car wiring, it's best to take your Explorer to a qualified mechanic for diagnosis and repair. Ignoring a wiring problem can lead to further damage or even safety hazards.
