Troubleshooting the Trailer Wiring Reverse Light Wire on a 2005 Ford Expedition  

Question:

Expert Reply:

If the additional blue wire is a factory installed wire and not a wire ran by a previous owner, then the issue is typically caused by a blown fuse or a bad relay. On your 2005 Ford Expedition, there should be a 20-amp fuse for the trailer wiring backup lamps is fuse position 10. If that fuse is blown or missing, that could be the issue. I also recommend checking the relays in position R201 and R301.

If the fuses and relays are present and in good condition, it may be possible that the blue wire was not completed when it was initially installed, or the wire has been damaged at some point. At this point, you have two options, you can take your Expedition to a Ford dealer to have them inspect the circuit, or you can splice a new wire into the reverse light wire at your taillight assemblies on your truck.

The taillight assemblies can be removed by removing the mounting screws. Once you have the taillight assemblies removed, you will see several different wires. Have a helper sit in the driver's seat, engage the emergency brake, step on the brake pedal and put the truck in reverse. Using a circuit tester, like part # PTW2993, probe each of the wires to find the wire that only has power when the truck is put in reverse. If you do not want to probe the wires, you should be able to remove the taillight wiring harness from the taillights and you can probe the pins in the taillight wiring harness.

I have attached a troubleshooting video that demonstrates how to probe vehicle tail light wiring for you to check out.

expert reply by: John H

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