Over the door is a main and coach switch. Is the coach switch in the on position?
You could have a short somewhere that is tripping an automatic breaker.
If yours is like mine the main panel is under the fridge. Remove 12 volt fuses one at a time until problem goes away and that circuit will probably be your problem.
If no fix then
If it is a chevy chassis under the hood on the drivers side you will find a relay that looks like this
Tap it with a hammer and see if that fixes the problem. There is a big washer inside that corrodes. You can buy a new switch or remove the one you have and burnish the big washer which will make it work for a while maybe 6 months maybe 5 years.
