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have you check the fuse and the horn relay?

I have started a thread on adding additional horns. I am adding new wiring for mine. In your case you could follow my mounting guide and still use the existing horn wiring.
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you can take the existing connector off the horn by cutting the wires about two to three inches from the connector. Solder a Y from the connector to the new horns. If the horns are air powered then...