OK, you now know that your horn works, so Im sure all you have to do is connect the black wire to the pint you could see in your first pic, where it had originally broken off.
You probably dont need to add a second wire, and i will tell you why.
OK, Point C is the "signal" (to use your term) or -ve from the horn. Points A and B should both be earth connections.
What normally happens is that when the horn button is inside the Boss, Point A actually rubs inside the boss to give you an earth. This is usually all you need to complete the circuit. that way the boss acts as the second wire.
BUT
Sometimes this can be unreliable, so you can connect point B to a second wire, and attach that wire to the boss with a screw or bolt. This can give a more reliable earth, but most people dont use it, and you shouldn't need to either, once you fix the broken black wire.
