That one wire in the column is the ONLY wire in the circuit. LOL. I pulled every inch of electrical anything out of that truck several months ago. I bought a (modern) harness from Jim Carter and am now starting to re-do everything.
After looking at that diagram and reading a few "tech tips", it appears that column wire (and the bearing it's soldered to) is actually "hot". Depressing the horn button actually grounds the circuit, making the horn toot.
horn techtip 
