Here's a weird one for you all... The fault is that only the right rear marker/running light does not work.

The truck is a 1988 F350 custom, diesel. Power brakes and steering, everything else is plain jane manual. Yes it's set up to tow, and electrically there is a well weatherproofed junction box behind the driver's rear wheel up on the frame rail where the towing connector attaches to the rest of the lights.

The bulb is a two-filament style, with one filament doing brake/turn signal and the other marker.

All other lights on the truck work in every combination, ie. with turn signals on and off, with brakes on and off, with markers on and off, and all other combinations I can think of.

A new bulb does nothing (else I would not have posted this)

Perhaps co-incidentally, my left turn signal no longer switches off after I've come out of a turn. Right side is just fine however. So clearly I've got a broken turn signal cam in the steering column.

Anyone that can post this section of the wiring diagram earns my undying thanks. Anyone else that has any ideas also earns my thanks. Winning ideas that are in my area also earn a beer or two.

Thanks

 

The schematic shows a common Brown wire feeds the filament for both rear parking lamp bulbs and also the license plate bulb or/bulbs.

The top right side of the schematic shows the wiring for a Cab and chassis.
The middle schematic shows the wiring with no factory rear bumper.
The lower schematic shows the wiring with a factory bumper.

So the Brown wire for some reason is not providing 12 volts to the right rear filament for the Park filament portion of the Park/Turn signal bulb.

It can't be a ground problem, (Black wire since you say the turn signal and brake light works on the right side). (Both filaments share a common ground).

Visually verify the socket is good and if it is, trace the Brown wire and look for an open.

Sounds like you may have an aftermarket trailer connector harness which plugs into the factory harness at the driverside rear.

 

Thank you for both the wiring diagram and the step by step. After the day I had changing my $*(#)@ fuel separator, my brain is cooked. Hopefully this job will go a wee bit smoother. The lower diagram matches mine, factory bumper and all.