I looked at my light this morning. The previous owner installed an after market light wire set for pulling a trailer. You know the plug in the bumper, so you can plug in trailer lights. There is no ground wire coming down to the frame. I looked at the wiring diagram in my chiltons manual and attached a wire to what should have been the ground. I applied the brakes then spliced the wire in and grounded it hoping for the light to come on. Nothing happened. I have no idea where to go from here. It's just stock lights. Incandescent bulbs. This is a work truck. All I want is the lights to work, so I don't get a ticket.

 

Magnethead said black is ground
brown with red tracer is right turn signal and brake (or Dark Green with brown tracer)
black with yellow tracer is running/marker/parking lights
Violet with black tracer is reverse
My light isn't like that. There are three wires going into the plug that houses the blinker, brake light and taillight. The three wires are Green with a Black Strip, Black with a yellow strip and Black.
I found the wires Magnethead mentioned under the truck. They might have been going to the passenger side light. I don't know.

 

Does the left turn signal work from the trailer harness?

 

My bad, i did read the wrong side.

Black/Yellow is still running/marker/parking
Violet/Black is still reverse.
Black is still ground.
driver side brake/turn is green/brown.

Do running/marker/parking lights work on the driver side taillight?
- If yes, it's a brake/turn issue.
- If no, it's a ground issue.

Both taillights plus the license plate light all share that same black/yellow wire. That wire splits apart at Splice #316, located on the driver side frame rail just forward of the spare tire carrier, in the vicinity of the shock mount.

The Green/Brown wire is a dedicated run all the way from cab/chassis Connector #352 located in the driver fenderwell, behind the splash shield.

I have seen in the past, that the Plug-N-Play trailer harnesses can become de-pinned or corroded or otherwise fail internally, causing a no-lamp.

Both bulb filaments ground to the single ground wire, and are supplied individually by the two different wires.

 

So you had juice on the black wire, not the black/yellow wire? If that's the case the previous owner probably didn't install the trailer harness correctly. I suggest removing the aftermarket trailer harness, then make sure there aren't any cut wires. If you see any clips that look like this, remove those horrible things and correctly resplice the wire at this point. You can also check with the online factory parts sites, you may be able to get a replacement harness for the rear lights.