Tail and brake light gremlins
 

Hazards work, tail lights work (in parking or headlights position).

With no parking/headlights on brakes will light both elements on DS and only one element on PS.

With parking/headlights on brakes will light both elements on DS and NO elements on PS!

I've checked common ground point, fuses, replaced bulbs, etc... The "no lights at all on the PS when braking with headlights on" is really throwing me! Should I look at replacing the bulb holder? Light switch itself? Other?

1980 model; 4-spd

Here a quick video link. You can see that when I turn my headlights on (license light comes on), all light ceases on the PS when the brakes are pushed.

It is probably not the light switch. If you have a test light or meter you should follow the voltage from source. (brake switch) back. Your problem sounds like is only the passenger side lights. I"m sure the voltage going to that side feeds both bulbs for brakes. There is probably a coupler in cab and one at rear lights. You can either take apart and clean and check voltage to ensure you find fault. The sockets probably have a ground so ensure both are clean and grounded properly.With one set of lights working I suspect brake light voltage is at the back drivers side but not getting over to passenger so I would start there. I do not have a wiring diagram but my ideas. There is a forum her where a gentlemen offers free wiring diagrams for specific years. I would search it and have him send to you for reference.

on diagrams looks like most of the wiring goes through the turn signal switch and headlight switch. probably one of those causing the problem . looks like there is multi wire connector by the steering column for the turn signal switch, might check it out for continuity.

Just to wrap this thread up and provide information for future form searchers… So I did my best to methodically go through all the possibilities and yet still ended up doing three or four things trying to resolve the problem.
1. Track down voltage through the main brown wire leading to the tail lights and confirmed that one tail light was not getting voltage through. So unwrapped wiring until I found the factory splice (noted some corrosion so cut it out and respliced/heat shrinked).
2. Added an additional ground wire directly to the passenger socket that was giving me the trouble – grounded to the frame on the passenger side similar to what the factory did on the driver side.
3. Still no luck although now I was getting voltage to the socket and felt confident in the ground as well, so replaced the socket with new on the passenger side… Lo and behold now the problem swapped to the driver side!
4. Replaced socket on driver side as well, and finally – success!

I believe my problem was simply a combination of corrosion in the factory splice within the wiring loom and worn out sockets… Electrical on these beasts can drive you mad.

https://cimg0.ibsrv.net/gimg/www.cor...15f0ba6367.jpgNew tail light socket
https://cimg1.ibsrv.net/gimg/www.cor...2b00bfdc61.jpgAdditional ground added to passenger side