Tramp,

I have enclosed a link to some wiring diagrams for your truck. The site lists a '63 schematic that is very similar to your '64. In the diagram, it shows a second wire connected at the positive battery terminal that goes to the horn relay. I don't know if this could have any effect on your problem if it were left off.

This era truck was loaded with bulkhead connectors for the wiring, and those sometimes get funky and loose. Good luck and keep us posted.

http://rmcavoy.freeshell.org/Truck_gauges.html

Stuart