[Dave at Pelican Parts]

Pushing the button closes a circuit by shorting the wire coming up the steering column to ground. (The column is the ground.) This completes a circuit through the coil of the horn relay, which pulls that relay closed. The relay closing sends current to the horn, and the other side of the horn is grounded. Current flows through the horn, which makes noise.

The horn relay is one of the ones on top of the fuse panel. Check the wiring diagram for wire colors for that relay, then look for a relay socket with those color wires going to it.

--DD