I'll soon have onboard air, and my current horn doesn't work, so I'm thinking I should make another trip out to the old semi wrecking yard. Anyone know how these things work or what I'll need to grab? I am hoping it will be something obvious like an air fitting and a wire terminal, but I don't know. Are they electric solenoid actuated, or is there some kind of a spring valve? What kind of psi do they operate off of?