1. Check the Trailer Wiring Harness Connection:
* Disconnect the trailer harness: This is the most important first step. Completely unplug the trailer harness from your S10's wiring. This isolates the trailer wiring from the truck's system.
* Test the passenger-side tail light: With the trailer harness disconnected, check if the passenger-side tail light now works. If it does, the problem lies within your trailer wiring harness connection or the harness itself. If it still doesn't work, move to step 2.
2. Inspect the Trailer Wiring Harness:
* Visual inspection: Carefully examine the trailer wiring harness connector and its wiring for any signs of damage, corrosion, loose wires, or broken connections. Pay close attention to the wire leading to the passenger-side tail light.
* Continuity test (if you have a multimeter): A multimeter can help pinpoint breaks in the wire. Test the continuity of the wire from the connector to the tail light socket. If there's no continuity, the wire is broken somewhere along the route.
3. Check the Tail Light Socket and Bulb:
* Inspect the socket: Even if you've checked the bulb, make sure the socket itself isn't damaged, corroded, or loose. A poor connection at the socket can cause intermittent or complete failure.
* Replace the bulb (again): While you've mentioned checking the bulb, it's worth replacing it again to be absolutely sure it's not a faulty bulb causing the issue.
4. Investigate the S10's Wiring (If Step 1-3 didn't solve it):
If the problem persists even with the trailer harness disconnected, you have a problem *within* the S10's wiring. This is less likely given the circumstances, but still possible:
* Ground connection: A poor ground connection for the passenger-side tail light circuit could cause this. Trace the ground wire from the tail light back to its connection point. Clean any corrosion and ensure a good, solid ground.
* Wiring harness issue in the truck: This is a more involved process and may require tracing wires within the vehicle's wiring harness, which is difficult without a wiring diagram. Look for any signs of damage or chafing to the wires leading to the tail light. This is best left to a qualified mechanic unless you're experienced with automotive electrical systems.
* Fuse (again, but be thorough): Double-check your fuse box again. Sometimes a fuse might look okay but still be blown internally. A multimeter will confirm if it's truly intact. There might also be a separate fuse specifically for the trailer wiring that’s blown.
Important Considerations:
* Wiring Diagram: A wiring diagram for your specific year and model S10 will be invaluable in troubleshooting. You can often find these online through forums dedicated to S10 trucks or from repair manuals.
* Safety: Always disconnect the negative battery terminal before working on any vehicle's electrical system.
If you're not comfortable working with automotive electrical systems, it's best to take your S10 to a qualified mechanic. Improper wiring can lead to further damage or even fire hazards.
