You may have a switch on the dash that over rides the air horn , and allow an electric horn instead. If you have an electric horn working, at least you know the horn button is ok.
You then will need to look for an air solenoid, where the line from the horns hooks to the chassis, air system, and check for power when the button is pushed and good ground.
What chassis , is your Bounder built on ?
My Freightliner has an air manifold , with the solenoid attached under the drivers feet next to the gen set.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.