* Faulty Turn Signal Switch: This is a common point of failure. The switch itself might be internally shorted or worn, causing a constant power feed to the taillights even when the turn signal isn't activated.
* Wiring Harness Issues: Even with the trailer wiring fixed, there could be other damaged or frayed wires within the main vehicle harness leading to the taillights. Look for broken insulation, chafing against metal parts, or corrosion, especially near the connection points.
* Grounding Problem: A poor ground connection for the taillights can cause erratic behavior. Ensure the ground wires for both taillights are clean, secure, and making good contact with the vehicle's chassis.
* Multi-Function Switch (Turn Signal/Wiper Switch): Similar to the turn signal switch, this component can fail and cause cross-communication between functions, leading to taillight problems.
* Central Turn Signal Module (if equipped): Some vehicles have a dedicated module that controls the turn signals. A faulty module could be the root cause. This is less likely on an older truck like yours, but still a possibility.
* Tail Light Assemblies Themselves: While less likely given both lights are affected, a short or internal fault within one or both tail light assemblies could be contributing to the problem. Carefully examine the internal wiring of the taillights.
Troubleshooting Steps:
1. Test the Turn Signal Switch: Use a multimeter to check the voltage at the output wires of the switch in different positions (off, left, right). You should see a change in voltage when the switch is moved.
2. Visually Inspect Wiring: Carefully trace the wiring harness from the taillights back towards the turn signal switch, looking for any signs of damage or corrosion. Pay close attention to areas that experience flexing or movement.
3. Check Ground Connections: Clean and tighten the ground connections for both taillights. You might need to use a wire brush to remove corrosion. Test the ground connection with a multimeter.
4. Test the Tail Light Circuits: Use a multimeter to test the voltage at the taillight sockets themselves. Compare the voltage with the ignition on, the running lights on, and the turn signals activated. This will help pinpoint whether the problem is before or after the socket.
If you're not comfortable working with car electrics, it's best to take your truck to a qualified mechanic or auto electrician. Diagnosing electrical problems can be tricky, and incorrectly fixing them can lead to further damage. Remember safety first – disconnect the battery negative terminal before working on the wiring to avoid shocks.
