I know this vehicle is an E350, but thinking the F350 is similar.

06 E-350 Short story, running lights were working fine and the rear lights just stopped working. Front parking lights, headlights, dash lights work fine. Dimmer works as well.

The first challenge is that it is not just an E350, but a conversion in to a bus made by Supreme.
I checked the schematic on All Data and the circuit shows just a single wire coming off the pin 5, center pin on the head light switch, that is obviously working, the schematic shows the rear lights being connected to the same circuit somewhere in the harness and not at the switch.

To make matters worse, it is a modified bus made by supreme. I found that there is a relay where the schematic does NOT show one in the driver foot well. Maybe this is just a tap for other lighting which in the cab seem to work fine but the factory taillight harness does run down the frame and also the trailer plug.

Hoping someone can help with any of the following:

1. Pin Out of the master switch plug? This is a pull type switch, 2 positions with dimmer/interior selection
2. Does anyone know where the rear lights tie in to the front light harness?
3. Know of anything that isolates the front verse rear?

I've checked every fuse in both factory fuse boxes and also the aux box that is not factory.

There are also running lights along the body that are also not working but not sure if they ever worked.
I pulled the rear lights, I have a solid ground on all of them. I checked the 12V and no voltage, I checked 12V to ground and there doesn't appear to be a short. I put 12V on the rear light, all of them lit up, both the bottom and top of the cab.

With that being said, all the rear lights are on the same circuit, have good grounds, bulbs are good.
Any feedback is appreciated, Especially before I just run new wires back from the switch and call it a day.

 

Had the same issue! I found source after 2 years of searching! Wiring harness that runs from drivers side of transmission over to passenger side, up past exhaust manifold. Found shorted wires, melted. Pulled and repaired wiring, placed protective heat shield on harness and replace. No issues! Now.