Look for a 3/4" round silver unit in the fuse box or in the signal flasher box, ( I can't remember). It looks like the starter for an old florescent light fixture. It's a thermal device with a contact that turns off after it cools down for about 20 seconds. Sometimes those contacts stick. This unit was used way back to the early '70s and they have an extremely low failure rate, so almost any one you find in a salvage yard is likely to be good.