When I got my '76 CB750, the guy I bought it from had mounted new blinkers, but they still didn't work. I replaced the flasher, but no dice. Normally, I believe the lights are supposed to be grounded to the frame by mounting. There was only one wire coming from the blinker. when we hooked that to the proper hot inside the headlight, it came on but didn't flash. We ended up running a separate wire to ground it to the frame (ran a wire from the pivot bolt inside of the headlight to a mounting bolt on the speedometer, then hooked the blinker ground to that) to make sure the light was grounded. Then it worked!
All this backyard engineering is why each of our bikes are unique. Regardless of whether our solution was right, doesn't really matter. It was a ground problem, and we grounded it!