No they're 6 v
Its simpler than that I believe- the horn should have power whenever the key is on and the battery is alive. The horn button switches the ground(negative). So you will read positive six volts at the horn even without pressing the button. But there should be no current flow(and heat buildup) unless the button is pushed. So the button is shorted to ground or the wiring is wrong or the wire is shorted to ground.The short is not in the horn. I suspect it is in the mess in the headlight shell. The horn should not be getting voltage without the button being pushed. It could just be that the wrong wires are connected in the headlight. Whacking the horn is to dislodge any insects, dirt or rust which are preventing the diaphragm from vibrating as it should. Are you using the handle from a #3 Phillips? It should be pretty easy to identify the wires from the horn button and from the horn and see if they are properly connected in the headlight bucket. Check it against a wiring diagram, it is a pretty simple circuit.