Right! I'ts been so long since i worked on the lights i completely forgot about that.. I think the lack of a headlight was the original source of my problems with tail lights burning out.
Current to the tail is too high without a headlight since the headlight and all other lights on the circuit are just like resistors, absorbing "overvoltage".. An original, complete brand-new bike with all lamps operating has no problems with burned bulbs.
I remember going through the whole system after studying the wiring diagram and rewired some stuff.. I may have ended up completely isolating the tail lights from the headlight circuit.. can't recall the details but i spent a week on it..
Still, old tail lamps you find in a junkyard bike easily pop.. the elements wear out as they get thin with age. Any blacking coating inside a bulb's glass is an indication of it. New 12volt bulb replacements won't hurt, imo.