Trailer Wiring Harness Troubleshooting for a 2008 Scion tC

Expert Reply:

Check to make sure that the half of the harness with the yellow wire is run to the left hand side of the vehicle. If the two sides of the harness, part # 118484 are swapped, you will get improper signals to the box.

If you are sure the ground for the black box is a good ground, than I would check the trailer lights. Check for any loose connections or frayed wires and definitely check the grounds for the lights themselves. Some use ground wires, some use the hardware of the light itself to ground the connection. Make sure these connections are good.

Finally, check the harness function without the trailer attached. Use a circuit tester and connect to the white ground post on the connector and test each post of the connector at the end of the car. Brown is tail light is should be active when headlights are on, yellow is driver side brake and turn signal, and green is passenger side brake and turn signal. If the connector functions properly without the trailer attached, then you know the problem is on the trailer.

How did you verify the ground? If you are using a common ground point, there may be interference from the other ground wires and the harness may need to be grounded by itself. Write back if you do not find the problem and we can go from there.

expert reply by: Patrick B