[alexiskai]

The horn works regardless of whether the key is on, so the ammeter movement you observed is normal and would seem to indicate that the wiring between the horn and the rest of the car is OK. The yellow wire provides power all the time. The blue wire routes to the horn rod; when the horn button is pushed, it closes the circuit, thus causing the horn motor to turn and sound the horn.

If the ammeter is only deflecting when you push the button, that at least means the power input isn't grounding to the frame, so that's good (that's a common problem).

You can test the horn while in the car by disconnecting the horn from the car battery and then simply connecting leads to each terminal.