I too will go with the flasher controller unit. If it goes then none of the turn signals will work. It's a little silver, usually, canister, usually cylindrical, maybe an inch + diameter and maybe two inches long. It's under the dash somewhere. They're generally not to hard to find. It's a plug in unit. Wobble it out and change it out for a new one. I use a heavy duty flasher because it also controls the cabover camper I carry, thus giving me extra lights to flash. The 4 ways may have a dedicated flasher, all their own, as a safety measure. So, there may be two flasher controllers under there. They're probable, maybe, identical. If so, switch them and see the results.
