If its like most horn systems on these MVs, I take a guess and say its a chaffed or worn wire in your steering column. Your horn button is on your steering wheel, right? If so, somewhere the wire comes out of your steering box for your horn. This is a common place for wires to break for horns. That is just where I would start looking for a wire that is worn through the insulation and grounding out, causing the horn to sound.