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Stock horn had battery power to one terminal (call it A) at all times. Actually, both terminals (A and B) should read battery power when the horn button isn't pushed. Inside the horn there is a normally closed switch connecting terminals A and B. Terminal B has a wire that goes back into the steering column and attaches to the brass ring at the top of the steering column. The steering wheel has a brass contact that connects one side of the switch under the steering wheel cap to the brass ring.

When the horn button is pushed it provides a ground for terminal B which activates an electromagnet inside the horn, moving a diaphragm in one direction and compressing a spring. Once the diaphragm moves a short distance it opens the switch in the horn, stopping electrical flow, turning off the electromagnet and allowing the spring to push the diaphragm back to its original. The process repeats until you let go of the horn button.

Quick troubleshooting steps:

You've already confirmed power to the horn, so ...

1. Confirm power to the other horn terminal. It should be there if the horn button is not being pushed.
... If power, skip to step 2.
... If no power, the contacts or wiring inside the horn are defective.
...........SOME horns can be opened and the contacts could be cleaned with VERY FINE abrasive.
...........Reassemble and try again.

2. Remove one wire from the horn. Check the wire for power.
... If power, reinstall the wire to the same terminal which we will call X.
... If no power, reinstall the wire to the same terminal which we will call Y.

3. Use a short piece of wire, a screwdriver, or practically anything metal to short terminal Y to ground on the engine nearby. The horn should HONK LOUDLY.
... If the horn honks loudly, the problem is between the terminal B wire and the ground provided by the horn button.
... If the horn makes little or no noise the contacts or wiring inside the horn are defective.
...........SOME horns can be opened and the contacts could be cleaned with VERY FINE abrasive.
...........Reassemble and try again.