with headlights off and the hazard button is pulled, no hazards lights come on.
with the headlights on and the hazard button is pulled, right turn signal/hazzard stays on, no hazards blinking.
maybe need new button? all light bults change with new flasher

also had same problem with 81 C-10. only R/S headlight would turn into hazards.

I would guess you have a bad ground along the line.
Start checking and cleaning them ALL. Time well spent for future problems, too !