seca 2 electrical problems

I have a Seca 2, 1993. The tail light has gone out recently, but after attempting to replace the bulb, I discovered that it still didn't work. With both bulbs the brake light continues to work. I feel that this is some kind of wiring issue, but after thumbing like a 3rd grader in graduate school through my service manual, I don't think I know the first thing about checking the wiring. I have a voltage meter, and know how to to use it. Can someone give me a nudge in the right direction of which parts I should check, and how?

A second problem that I believe to be unrelated: occasionally when I go to turn the bike on, I put the bike in neutral and hit the ignition switch, theres a click and the lights go out. Nothing works after that. So I jump it, ride for awhile, or pull the battery out to trickle charge (where it registers a full charge already) and it works fine for a couple weeks then repeats. It seems to be completely random when it will happen. I have checked the battery's voltage when its off, on before starting, while starting, and idleing, and they all are fine. I think that it is a problem in another part of the bike, but I don't know what or how to check that, can someone advise?