First find out whether it's the horn or the wiring by locating the horn and disconnecting the wires. Get a friend and use a tester screwdriver to check whether it is live when your presses the horn.
If it is live then you just need a new horn - get down the scrapyard and find a replacement.
If it's the wiring check the fuse. If it's not the fuse - then something more tricky.
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