My car is old enough that it does not have a horn relay. Being a '76 yours might, check the wiring diagram to see.
Check your wiring diagram to confirm the information below. My comments are based on generic Lucas color codes and may not be accurate for your car.
The horn circuit is always "hot" with power supplied from the fuse box via a purple wire. If your car does NOT have a horn relay, that purple wire goes directly to one side of the horn. The other horn terminal will have a purple black wire that goes to the horn pushbutton. The other side of the horn pushbutton provides an earth/ground connection. Push the button, complete a path to earth, the horn sounds.
If your car has a horn relay the wiring will be similar but a little bit more complicated. The purple wire will still be hot but it will connect to the horn relay, not the horn. A purple/yellow wire will go from the relay to the horn with the other side of the horn (or horn terminal) connected to the car's chassis/earth. The purple/black wire will run from the relay to the horn pushbutton. The purple/black wire and horn pushbutton still provide a path to earth/ground but this time the switch closure energizes the relay instead of directly energizing the horn.
Therefore, if you are using a multimeter to troubleshoot this you should find 12V anywhere along the purple wire and the purple/black wire up to the pushbutton switch. If you have a relay and purple/yellow wire, that wire will only be "hot" when the horn pushbutton is held in.
Again, this is all generic Lucas information and you should confirm what I am saying by comparing it to your car's wiring diagram.
