Al,
The air horn solenoid valve has power at all times and it is activated when the horn button completes the circuit by furnishing a ground. The most common problem with this valve is corrosion, especially with roof mounted air horns that let water drain down to the valve. You can some times take the valve off, and spray a good penetrant into the ports, tap it on a bench, and activate it a few times to get it working again. If you are getting water in the valve, you can blow the horn on a regular basis, especially after driving in a rain, and this will keep the valve clear and prevent corrosion.
Gerald