1. Bulbs: This is the most common cause. Even if one side works, it doesn't mean the other bulbs are good. Replace *all* taillight bulbs (including brake lights and turn signals, if those are also out) – both sides. Make sure you get the correct wattage bulbs. Check the bulb sockets for corrosion or damage.
2. Bulbs Sockets/Connectors: Examine the bulb sockets carefully. Corrosion can prevent a good connection. Clean them with a wire brush or contact cleaner. If the sockets are damaged, replace them.
3. Tail Light Switch: This switch is usually located on the brake pedal assembly. A faulty switch won't send power to the taillights when the brakes are applied, but may still work for running lights. It's possible the switch is failing intermittently. You'll need to test this with a multimeter (explained below).
4. Wiring Harness Connections: While you say you checked the wires, thoroughly inspect all connections in the harness leading to the taillights. Look for broken wires, loose connections, or corroded connectors. Pay close attention to the area where the harness enters the body and the taillight assemblies.
5. Ground: A poor ground connection can prevent the taillights from working. Find the ground wire for the taillight assembly (usually a bare metal point connected to the body) and clean it thoroughly. You might need to add a supplemental ground wire for a more reliable connection.
6. Multi-Function Switch (Turn Signal Switch): If your turn signals or brake lights also aren't working, this switch could be the culprit. These switches often wear out over time.
7. Body Control Module (BCM) or related computer (if equipped): While less likely in a 1991 model, a faulty BCM could potentially affect taillight function. This is a more advanced troubleshooting step.
Troubleshooting with a Multimeter:
A multimeter is essential for more in-depth diagnosis. You can test:
* Bulb Continuity: Check if the bulbs are intact by setting your multimeter to continuity mode (usually a diode symbol) and testing the bulbs.
* Voltage at the Bulb Sockets: Set your multimeter to voltage mode (DC voltage). With the lights switched on, check if there's voltage at the bulb sockets. If there's no voltage, the problem lies upstream (fuses, switch, wiring).
* Voltage at the Tail Light Switch: You can test the switch to see if it's providing voltage to the taillights.
Important Considerations:
* Wiring Diagrams: Obtain a wiring diagram for your specific 1991 Chevrolet Silverado. This will help you trace the circuit and identify potential problem areas. You can often find these online through forums dedicated to classic Chevy trucks or repair manuals.
* Safety: Always disconnect the negative battery terminal before working on electrical systems to avoid short circuits.
If you're not comfortable working with car electrical systems, it's best to take your truck to a qualified mechanic for diagnosis and repair. Trying to troubleshoot electrical problems without proper knowledge can be dangerous.
