The horn is grounded to the sheet metal by the bolt that mounts it to the radiator support. Check to be sure there is good clean metal there for the horn mount bracket to bolt and bolt to body sheet metal. If that doesn't cure the problem, then remove the horn and test it with a 12 volt battery to be sure it will consistently work. If the horn checks out OK, then work backwards along the wires to the fuse box and finally to the steering column wiring using a jumper wire as you go.
