First make sure the horn itself is functioning. A lot of times they get frozen internally from lack of use.
Pull the harness connector off the horn and run a jumper from a 12VDC source. Make sure the mounting bracket is tight and the bolt threads are clean - thats the ground path.
If it doesn't make any noise - sometimes you can feel it vibrating but not enough to honk - remove it from the radiator frame. Squirt some PBlaster or similar down the trumpet and let it sit overnight. Try it a again. If it still won't sound, get another horn.
If it does beep with a 12 volt jumper supplying the power - it means the horn is good. You have a power supply or control circuit problem.
First on the power supply, check the horn fuse.
If it's good, look at the control circuit. Try a new horn relay - they're only about 3-4 dollars.
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