Wiring is a mess to begin with. My passenger tail light doesn’t work and I have turn signals and brake lights all the way around but my passenger taillight is a no go. The tail light and the brake light are the same bulb so I’m guessing wiring. Anybody got any ideas?

 

I have an '85 W350, my guess is yours, like most, has had a wiring harness for trailer cobbled/chopped into it at some point. Good guess to start, those crap crimp connectors lazy people use to tap a harness are criminally useless for keeping corrosion out.
See if the wiring diagrams at the top of this page match your wire colors, my guess is they do. Start tracing back from the tail light with a test light until you find power. If you have power back there you have a ground issue. The wiring on these trucks is stone axe simple, but 40 yrs old. Good luck.

 

Either tired old lamp sockets or weathered splices. Can buy new lamp sockets cheap and easily.

I put in LED versions of the 1157 lamps, and tractor lights for backing up.