Where to start looking....

I can tell you from experience that my 67 has always been finicky about the ground of the bulb socket to taillight housing. It has also been really finicky about using brass housing taillight bulbs to get a good ground for the socket. The newer nickle plated bulbs have a hard time gaining a ground from the socket/housing ground. Since you mentioned that you just did your taillight housings, maybe it is something in this area??

I also had a problem with passenger taillight and both brake lights working at all. I took a piece of info offered here and pulled my steering wheel and cleaned and debentures the contacts of my turn signal cam area, where all things taillight seem to happen! Cleaned and lubed that all up and the taillights and brake lights work like a charm now??

Is any of this your culprit?? I don't know, but its a good cleanup and reset for those often neglected areas and worthy of attention anyway!! I will mention that neither of our related problems had any effect on my battery charging system!!

Hope you can track it down...it sure is nice out today!! Geof