It's always something....I put a completely new wiring harness (two of them, actually, the main one and the headlight one) within the last year or so. Then installed turn signals. The switch (original Lucas) is always a bit iffy but if cleaned and lubed regularly it works "okay."
Recently the left rear signal stopped working. Had to pull the tank and trace the power supply from the right bar switch all the way back to the turn signal. Had power the whole way; must be inside the signal lever assembly itself. Started to dissassemble it and it gave up and began working again. Never could figure out just what I 'jiggled' enough to make it work, but if it happens again I at least know where to look.
Went on to valves, petcocks, and other stuff I needed to do while tank is off (is it just me or is getting the tank on a complete PITA - of course, my front tank studs come out with the nuts, making life a bit tougher...)