[gophersnake]

For whatever help this may be to mole2000's troubleshooting, here's some info taken from the official GMC wiring diagrams for a '68 CM 2500:

The engine compartment diagram shows the horn grounded somewhere very close by, probably through its mounting bolt. It gets its power through a "12DG" (#12, dark green) wire from one of the firewall connectors.

The cab diagram shows another 12DG wire between the firewall connector and the horn relay (which looks like it must be under the dash, not under the hood).

The horn relay gets its power through a 12R (red) wire from a 5-wire junction not far away. The wires at the junction are always hot; they're powered from the fusible link near the battery and carry power to the fuse block, the headlight switch, and the ignition switch.

The horn relay is operated by grounding it through its third wire, a 20B (black). This wire is shown running to the C-shaped, 8-wire "directional signal connector" on or in the steering column. On the other side of the connector an 18B wire (perhaps part of the directional signal harness) runs to the horn contact. The other side of the horn contact is shown grounded somewhere nearby, perhaps to the steering column.

So when I press the horn button, that closes the circuit which energizes the horn relay, which sends power to the horn. Do I have that much correct?

Yes, that's the way it's supposed to work. It sounds as if your steering column may have been (a.) ungrounded and (b.) shorted to the horn relay wire so that when you touched it with the wrench you were in effect grounding the horn relay. That doesn't explain why the horn still worked normally from the button unless there was another ground path for it -- through the steering shaft, maybe.

I'm with VetteVet -- check out all of the above with a meter (or something -- I prefer a test light myself, the kind with the spike at one end and the alligator clip at the other, because it's so much easier than juggling a meter).