Tail Light Wiring Help Needed

Grounds, grounds grounds.:D
I use a separate wire clipped to the bumper or a body bolt to check them, and touch the bare end to the bulb base where it goes into the socket while the light is on.
Grounds in the tail/brake lights area suck on these cars, and I'd bet you have some crossflow of power due to bad grounds.
I actually drilled the housings and ran a dedicated ground through the lens housings to the bulb sockets themselves because they were so sketchy about it.
Note that even if you have a for sure ground to the light assy, the actual bulb socket metal looses it's ground to the metal housing from corrosion between the two metals.

Went through that same thing not only on my 73 but the Wife's 72 as well.
Pretty much with ANY electrical problem on these cars check grounds first and you'll usually find that was the problem.