Jed,
Oldsub made a good point, and was probably right on target...I know little about '46 trucks! Your first theory was then correct, ground the horn and power it through your aux horn button. You might want to install a little inline fuse before the button. If the system is 6v and you choose to install a horn relay, the horn button is wired the same, except to the coil of the relay instead of the horn. The normally open contacts of the relay are wired between the battery or amp meter and the horn.
The horn relay can use larger wire and can be a little closer to the horn which gives a chance to pass more current...louder more responsive horn. The little button in the cab doesn't have to try and pass the load for the hungry horn. I went to the gallery and looked at your '46..beautiful, and what a story of discovery!
Didn't this truck have a steering wheel horn button? You could probably get it fixed up and working...much more convenient than a button somewhere else..honk honk
Stuart.
