hi seamus i had a similar problem with my 550k3 according to the manual the horn has a live feed (black wire) then the light green wire goes to the switch which when pressed goes to earth.with the later type incorporating the pa flasher works differently,the black wire supplies a poss supply to the switch, then to the headlamp via the white wire when pa switch pressed sideways, it earths through the headlamp earth.The horn button when pressed sends a pos feed from the black wire through the light green wire to the horn, the problem was on mine the other horn terminal was another black pos feed so it didnt work.the solution was to connect the second horn terminal to earth result beep beep!!hope this helps, Mick