I tried a search first knowing this must be fairly common but found nothing that seemed like my issue. A few weeks ago my horn (stock) stopped working. No apparent reason like water which seems to be the most problematic issue. I ended up getting a cheap aftermarket horn on ebay, took my assembly apart, hooked up the replacement, tested it and put it back together. Couple days later I went to use the horn and no sound. Not expecting anything I contacted the ebay seller and he sent me another at no charge. This morning I went to replace the horn, hooked it up to the terminals and it worked. That is until I completed assembly, then just a barely audible grunt. I figured I would try my stock horn just for giggles and it is now working and re-installed on the bike. Any thoughts besides blowing out any suspect water from what recommendations I already read?

 

could be a grounding issue.. the horns them selves are grounded to what ever they are screwed to. i have my horn on the front of the frame and i had to cut out a plastic/rubber bracket to mount it to so it doesnt ground out on the frame...

 

Check the ground wire between horn and frame. The horn does a lot of shaking. The wires inside the insulation can crack. While holding the horn button down, jiggle the ground wire and see if it has any effect on the sound of the horn.