This is actually on one of my 350 Twins but I know the wiring is basically the same for the early 70s bikes. The tail light will only burn in the second key position, not in the first (run) position. I've tried multiple tail lights and even a different ignition switch. On a, possibly, related note, the light I'm trying to run has an extra wire, three, vs. two. Yes, this is my first post. I will happily introduce myself. I'm Trace, live in beautiful western NC and my current projects are two 71 350 twins, basically done, and just starting on a 79 cb650z. Cheers.