assuming you have an original horn usually all it needs is cleaning of the armature where the brushes ride, oiling of the felts at both ends of armature (not too much!), adjustment of the tension screw and good clean & tight electrical connections. sometimes the trouble is in the horn button or light switch rather than the horn itself. got a dead horn right now on one of our As, I suspect it's in the button, haven't taken time yet to bypass the button and see if it works......