[dmjlambert]

The horn button's function is to short the metal post that is sticking out of the plastic tower on the cancelling cam to the steering column center hub. So, short the metal post to the steering column center with a screwdriver and you should get some sparks and the horn will blare. If you don't get that, slide the cancelling cam off and under it you will see a metal post sticking up from the turn signal area, and that would normally contact the bottom of the cancelling cam. Short that to the steering column center and see if that honks the horn.
If that also does not blare the horn, look up under your dash and see how that extra under-dash horn button is hooked up. It could be the wire going between the steering column wiring and the horn relay was disconnected from the steering column (to connect it to the under dash horn button instead).