First of all, place a test lamp between the 2 electrical leads in place of the horn. Push the horn button and make sure the lamp lights up. Sometimes "good wiring" isn't all it's cracked up to be.

Second, while you got the horn removed, give it a good shock from as plastic hammer or a blow to the bench. Horns are very simple devices, but they can get stuck if they aren't used all the time. If all else fails, there's an adjustment screw to get the most volume out of the horn. You can move that screw back and forth to get it going again.

Once working, to keep it working simply exercise it a lot. I toot the horn at least 15 times on each trip.

Hope this helps.