I have everything electrical working on my 1930 pi
The weakest link in the model A horn is the connections. Start at the horn and work your way back. Remove the motor cover from the horn and you will see where the two wires are connected. Remove both of those wires and take some jumper wires and wire your horn directly to be sure the horn is working properly. Ground one terminal and touch a hot lead against the other. If the horn seems to be working properly, clean the bullet ends with some sandpaper or a scotch brite pad and reinstall them. If this is working properly, check where the light rod makes contact with the contact button at the bottom of the steering column. The problem would be either there or at the buttom on top of your light rod.