1. Bulbs:
* Check the bulbs themselves: This is the most common cause. Access the taillights (usually requires removing interior trim panels). Visually inspect the filaments in both taillights for breakage. Even a seemingly intact bulb can be faulty. Replace both bulbs (even if only one is visibly bad) with new ones of the correct wattage and type. A 1991 Lincoln Town Car likely uses standard incandescent bulbs, so getting replacements is easy.
2. Fuses:
* Locate the fuse box: Consult your owner's manual for the location and diagram of the fuse box (often under the hood or dashboard). Look for fuses specifically labeled for the taillights, rear lights, or parking lights. A blown fuse will be visibly broken or burnt. Replace any blown fuses with fuses of the *exact* same amperage rating. Don't just put in a higher amperage fuse; this is a fire hazard. If a fuse blows immediately after replacement, there's a short circuit somewhere in the wiring.
3. Wiring and Connections:
* Inspect the wiring harness: Access the taillight wiring harnesses. Look for any broken, frayed, corroded, or loose wires, particularly where the wiring enters the taillight assembly. Pay attention to ground connections – a poor ground can cause lights to fail. Clean any corrosion with a wire brush and reconnect securely.
* Check the connectors: The connectors between the wiring harness and the taillight assembly are prone to corrosion and loose connections. Carefully inspect and clean these connections. You may need to use electrical contact cleaner.
* Trace the wiring: If you suspect a break in the wiring, carefully trace the wires from the taillight assembly back towards the fuse box to pinpoint the problem area. This might require some patience and careful probing with a multimeter.
4. Switch/Relay (less likely):
* Tail light switch: While less likely if other lights work, a failing tail light switch could be a culprit. This switch is usually located on the steering column. Testing this requires more advanced electrical testing tools (multimeter).
* Relay: Some vehicles use relays for the taillights. Again, this is less likely if other lights are functional but check your owner's manual or a wiring diagram to identify and test any relevant relays.
5. Grounding Issues:
* Ground points: A poor ground connection is a common electrical problem in older cars. Find the grounding points for the taillights and clean them thoroughly. A loose or corroded ground can cause inconsistent or no illumination.
Using a Multimeter (Advanced Troubleshooting):
A multimeter is invaluable for precise diagnosis. You can use it to:
* Check for voltage at the taillight sockets: With the lights switched on, the multimeter should show voltage (typically 12V) at the sockets if the wiring is good. No voltage indicates a problem upstream (fuse, switch, wiring).
* Check for continuity in the wiring: This tests if there is a complete, unbroken electrical path.
* Check the ground: A multimeter can confirm a good ground connection.
Important Considerations:
* Safety First: Disconnect the negative battery terminal before working on the electrical system to prevent accidental shocks or short circuits.
* Wiring Diagram: A wiring diagram for your specific 1991 Lincoln Town Car is extremely helpful. You can find these online (often on forums dedicated to classic cars) or possibly from a repair manual.
* Professional Help: If you're uncomfortable working with car electrical systems, it's best to take your car to a qualified mechanic. Improper repairs can lead to further damage or safety hazards.
By systematically checking these points, you should be able to isolate the cause of your taillight malfunction. Remember to replace components with parts of the correct specification.
