E450 Passenger side tail light an turn not working backup is

I've spent a couple of nights on this. Still struggling but I started thinking tonight that where I'm losing the wires is where it splices into the factory wiring harness. If I'm right I just need to figure out which factory wire is passenger side stop/turn. Tail light works! Any ideas on the factory wire I'm looking for?

^^^ Sounds like it ^^^ Your flasher "thinks" there are burnt-out bulbs so it's showing you a fast-blink signal. It just doesn't know the difference between burnt-out and dis-connected.

Not getting power with prick light so doesn't that rule out ground?

Back to front. I got stopped at the gas tank. No power to turn/ tail or running lights as checked with the prick light. Driver side works great though. I'm also getting a fast blinker on the dash and grill blinker when on to the right/pass side.

Which way were you tracing, Rear to Front, or Front to Rear, when you lost track of the wiring at the gas tank?

Your chassis has been cut and stretched to provide the wheelbase needed for a large Class C. Wondering if the wiring connections have failed there.

Stop and Turn are directly related, from Brake Pedal and Turn Signal Switches to the rest of the coach. But Tail/Running is a separate circuit. They do, however, share GROUND. Do you have continuity from the Shell of the Bulb Socket, to the Chassis?

EDIT: Backup is another separate circuit, sharing Ground with everything else. That might mean Ground to the lamp assembly is good, but it's still possible for the shell of the Backup lamp to be good and the one for Stop/Turn/Tail to not be grounded.

I followed the wires with the ice pick last night. I got to the fuel tank and it was still not working. I lost the wires at the filler neck but it was getting dark and I was getting frustrated. I'll take another look at it tonight. Or maybe just let quad van fix it when I get my new front end put on. Hehe

For the lights, have you pulled the lenses to inspect the Bulbs and Sockets? An "ice pick" test light is very helpful on testing like this. Better than a meter.

I got so tired of rear lamp problems that I replaced our original Bargman incandescent lights with Bargman LED versions. They have to be connected to the wiring supplying the originals. This eliminates socket problems as well as bulb problems. Direct replacement otherwise. Same screw holes. I changed screws to stainless just because I could.