Okay, here's the deal.
Someone at some point (my guess is when they put the Pontiac steering wheel in) rewired the horn. The horn is currently wired from the Fan (was the heater) to a button on the side of the steering column, to the horn. The horn is then grounded to the intake manifold.
So, I fired the old girl up and touched a wire from the positive terminal to the batt terminal on the voltage regulator and I just about crapped my pants.
Not only does the horn work, it REALLY works.
So, in closing, I'll have to rewire it back to the way it's supposed to be. I'll probably have a peek at the rest of the wiring while I'm at it. Just because I know who used to own the truck.
Thanks for all of your help!