I'm no bike tech, but being a sparky, it would seem to me to be a problem with the the flasher module. If the switch makes the indicator light come on, even when held in, they are doing their job, if the indicator lights go on at all, the wiring must be OK, not much else left except the flasher unit.

Just a guess... May be feasible that shonky wiring could cause the flasher module to operate incorrectly, but that seems like a long shot to me.