* Left Turn Signal Wire Issue at the Trailer Plug: The most probable cause is a broken, corroded, or poorly connected wire specifically for the left turn signal within the trailer wiring harness at or near the trailer plug. This wire is only activated when the trailer is plugged in. The truck's own lights work fine because *their* wiring is separate and intact.
* Trailer Plug Itself: The trailer plug itself could be faulty. A damaged or corroded pin in the left-turn signal position could prevent the signal from reaching the trailer. Inspect the plug for damage or corrosion closely.
* Ground Issue: A poor ground connection for the trailer lights can sometimes cause intermittent or one-sided failures. Make sure the ground wire for the trailer wiring harness is securely connected.
* Trailer Wiring Harness (Truck Side): There might be a break or short in the trailer wiring harness *on the truck side*, specifically affecting the left turn signal wire. This would be less likely than a problem at the plug, but possible.
* Faulty Trailer Light Circuit Fuse (Less Likely): While less probable since the truck's lights work, a blown fuse specifically for the trailer lights (check your owner's manual for the correct fuse) could be the culprit. However, it's unlikely to only affect one side.
Troubleshooting Steps:
1. Inspect the Trailer Plug: Carefully examine the plug for any bent, broken, or corroded pins, especially the pin corresponding to the left turn signal. Clean any corrosion with contact cleaner.
2. Test the Wiring at the Plug: Use a test light or multimeter to check for continuity (a complete circuit) on the left turn signal wire at the trailer plug when the left turn signal is activated. Compare it to the right turn signal wire.
3. Trace the Trailer Wiring Harness: If the problem isn't at the plug, carefully trace the trailer wiring harness from the plug back towards the truck's main wiring harness, looking for any broken or damaged wires.
4. Check Fuses: Look in the owner's manual and check the fuse(s) related to the trailer lights.
5. Ground Connection: Inspect the ground wire for the trailer wiring harness. Make sure it's securely connected to a clean metal surface.
If you're not comfortable working with car wiring, it's best to take the truck to a qualified auto electrician. Improper wiring can lead to more significant issues.
