Bongo Fury

My mechanic tells me he has checked the horn unit independent of the switch and it works. He has checked the push switch independent of the horn and it works. The horn however does not work when the switch is pressed. He says he can't fix it because he can't find which wire (s) to check. Can anyone help?

Most likely the relay which is within two groups of three (six total) between the main battery and the offside headlamp.