1Fine39,

Welcome to the Stovebolt. I have enclosed a link to a '39 truck wiring diagram. It shows the horn as wired in the conventional manner which is 'hot' all the time.

The button in the steering wheel supplies a ground for the horn which completes the circuit and makes it toot!

This early system had no horn relay.

http://chevy.tocmp.com/chevywiring/pdf/39truck.pdf

Stuart