I went through the same situation with my '79 r65 -it was the relay, I bought a new one & all was well... for awhile ,then the next relay did the same thing. Because of the corrosion here in hawaii, i think, relays tend to go south quickly. Anyway, I just jumpered two of the sockets on the relay plug & have been o.k. since then.. the light is on when I start the bike. I keep a new relay in a plastic bag under the seat, don't know why...
Bill Conquest