I had a similar problem with my '42 half ton. It had been converted to 12 volts and had an appropriate after market switch. This switch, as well as the original, have a thermal circuit breaker. What happens is the contact gets dirty or corroded and normal use causes heat, which then opens the circuit, until it cools down again. Try cleaning that contact.
I think the heat is right there (don't reach up under the dash and feel for it or you will get a nasty burn).
