Do you have an ohm meter?  Check the continuity of the switch, then look for power in and if the ground is good.  If the switch is good.  power in and a good ground its the horn...otherwise its one of the above

Try some contact cleaner

Apply 12 V to the horn directly. No buttons, nothing, just a wire, straigt from a hot wire. Or, use a test lite to see if you have V to the horn, if you dont have a V meter.