Can anyone tell me how the brown power wire is routed to all the rear lights? On my truck (79'), from the main harness connector under the truck I have 2 black/red wires, one for each back up light, 1 yellow wire for the drivers side taillight, 1 green wire for the passenger side tailight, and then the brown wire is soldered in one place to 3 different brown wires with one for each tail light and 1 for the plate lights. This can't be correct. The passenger tail light is dim at all times. When the lights are on and I press the brake pedal the drivers side works correctly but the passenger side turns off completely. Am I missing some connectors for the brown wire? The PO was, and I use the term loosely, an amateur electrician.

 

That almost sounds like a ground problem, and here's why. On each side of the truck, the tail light and brake light source power from a different circuit, but both filaments in the bulb share the same ground. If the resistance in the ground path is too high, the voltage across this resistance increases as more current goes through when you have the running lights on at the same time you press the brakes. If that voltage goes too high, the ground is no longer at zero volts, which leaves less voltage across the bulb filament. If that voltage is small enough, you won't see any light. This is commonly referred to as "ground shift" because the grounded side of the bulb filament is no longer at the same potential as the electrical system ground.

Let me know if I have not explained this clearly. But it is a ground problem, and 9 times out of 10 the ground problem is within the socket itself (just from experience). If your bed is not grounded, you will experience the same problem. Remember, although the bed is bolted to the frame, the frame itself is not grounded through the engine mounts because the engine mounts through rubber. There is a separate ground strap running from the cab to the frame, and another ground strap going from the cab to the engine block. So your tail lights get their ground through the following path: (1) negative battery cable to engine block, (2) engine block to cab, (3) cab to frame, (4) frame to bed, (5) bed to socket.

 

I reinstalled/fixed all the grounds from the lights but the possiblilty of the box not grounding properly did occur to me. But what about the brown power wire, how exactly does it connect to each light? I don't think one wire should be soldered to 3 other wires, I didn't think electricity could be split that way.

 

Brown wire is for parking illumination use only. It connects via splices that are buried in the harness, I think. I'll have to confirm it in my wiring manual.