air horn setup?

Ok picked up a cheap air horn set up cane with horns, comp, pressure switch, small tank, and relay but dude couldn't remember how he hooked up the relay and I dont want to blow any more fuses or burn up any wires installing, I was planning on hooking up to factory horn button if thats possible

The relay is grounded through the horn button which supplies the horn relay ground to the stock horn normally as well. Once that circuit is completed the positive side of the relay feeds power to the horn. So you have to feed a positive source "in" to the relay and "out" to the honker. Fuse the positive side into the relay with a 20 amp and you should be good to go with your compressor.
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The pressure switch is there to prevent over charging the tank. I've never done one of these so I couldn't tell you how to wire that in. But I'm thinking it probably cuts current to the relay at a predetermined limit. So it probably goes into the negative side of the relay. Cut the ground and the relay shuts off, that sort of thing.
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You probably can get some additional help at

Hope this helps.