I would disconnect the horn and see if the fuse still blows. If so, then there is a short in the wiring somewhere, you'll have to trace it out. If not, then test the horn itself, perhaps it is shorting out. There is also a diode on the horn relay, and when it fails it can lead to the horn sounding on its own. I can't think of a failure mode for this diode that would lead to blown fuses.