You will probably have to pull your steering can off. At the bottom of the can there is a race that maintains contact with a plate as the steering wheel turns. When you press the horn button, that completes a circuit to ground causing the horn to blow. The wire that contacts the plate behind the can may be grounded or the wire that runs through the can may have fallen off causing it to go to ground. If those wires touch metal anywhere, the horn blows.
