The ammeter may be a good guess. All the things you mention attach to the ammeter, the headlight switch and/or the ignition switch. You can find the wiring diagram at:
http://chevy.oldcarmanualproject.com/electrical/wiring/pdf/51truck.pdf Since it is a pdf you can rotate it and enlarge it, it is pretty easy to read.
I'm a little perplexed about the light switch behavior, but my approach would be to check the wiring according to the diagram. Assuming it worked correctly before and no rewiring was done or disturbed, it may be a non-logical cause: such as a loose connection or missing ground. Good luck.
don
