Two points...
1) Yep - this is the wrong place. Should be in OEM tech. (I don't jump on you because you're already thinking you're doing something wrong - so I'm letting you know what would be right...)
2) The horns, on most vehicles, are down front somewhere near the bumber. I think they're in the front corners on my rigs - but I've got RENIX/AMC-spec (1987, 1988, 1989, and 1989.) Still, it's a good place to start looking - I think I usually see them when I have the front bumper off for some reason.
I've got to re-arrange things under my hood anyhow - I need to make room for a train horn, once I manage to find one. I'm getting tired of dealing with all these idiots out here, and I'd like to make them have to change their shorts a little more often...
Anyhow, try sliding under the front end of your rig, and looking in the front corners. There are probably some rubberised splash shields under there behind the bumper, so push them out of the way and look behind them.
5-90