Considering the current draw of a horn, one should always use a relay.
That being said, the horn has a single hot (12+) wire that travels from the horn through the steering column to a spring loaded carbon button located in the steering head (where the steering wheel attaches to the steering column shaft).
When the spring loaded horn button is depressed a slip ring on the steering wheel, makes contact between the carbon button and ground, completing the circuit and sounding the horn.
