I have a 1984 E-350 Diesel.
It's been sitting up for a while, and now the left brake light isn't working. The other lights are fine.

As best I can tell, the left brake light wire is black with a red stripe while the right one is a black wire with orange stripe.

As a quick fix, I made a 6 inch jumper wire between the two black wires at the back end of the van.
When I tested it, the left brake begain working, but the right one stopped.
I pulled the jumper, but the left kept working while the right wouldn't.
I taped over the spots were I'd removed the insulation and pushed the wires back up behind the bumper.

Well, the right brake started working again, but the left is out again.

Any ideas?

 

sounds like a poor ground...check by clipping a jumper wire from where you are sure you have good grounding.Could also be a defective socket in the light not working.

 

All electric travels one way unless a short. Check for shorted dual filament bulbs and look for a shorted or grounded wire for the left side. Jumping one side to the other won't help - ground only to a clean or soldered connection.