I would guess that its either a bad contact somewhere or the horn units gone belly up. Check the horn unit first with a direct 12v supply to ensure that's working, lf not there's your answer. If it is then work back from the unit cleaning up all the contacts along the way (including the mounting as this acts as the earth to the body). If it's an intermittent fault then thats another story, you'll be pulling your hair out as they're always a nightmare to trace. Good luck in your quest young Jedi..........