The right rear tailight on my 74 F250 is never as bright as the left one. When I'm driving with the headlights on, and I put on the brakes, the taillight goes dark. I have examined the lightbulbs themselves and their contacts in the sockets, and everything seems okay. I even made sure the two ground terminals in the recess are contacting clean bare metal. Someone told me that on 73-79 Fords, it usually can be traced to the turn signal switch of all things (?). I would like some input from the forum and how I might go about testing the turn signal switch to see if that is it. New switches run from $70 - $115 depending on the vendor. Ouch!

 

It still sounds like a grounding problem. Just to make sure, get a scrap piece of wire long enough to go from the battery negative terminal all the way back to the truck(just lay it on the ground). Take the light apart, strip the end of the scrap test wire, and stuff it down beside the bulb while the bulb is still in the socket. Just try to get it to make contact with the brass part of the bulb(if you have a helper, you can just hold it against while someone operates the brakes). Hook the other end of the wire to the negative terminal of the battery. Then turn the lights on, and hit the brakes. If it's a grounding problem and your test wire is making good contact on the outside part of the bulb, your problem with go away.