Sounds like the air tank is missing or disconnected. On my '89 Elandan, there is an airtank mounted on the center side of the grill support which is next to the chassis battery. The air tank has the solenoid for the air horns mounted on top of it. The system on the Elandan is similar to the Hadley Ambassador Air Horns
. Take a look at the picture in the link - it shows all of the components.
My tank has a bunch of holes in the bottom. Since it can't be drained, it rusted from the inside. You can test the airhorns by hooking up low pressure compressed air to the system. I used about 40psi to test, and used a blow gun so that not all the air was forced through the system.
The yellow line goes compressor->tank mounted solenoid->airhorns.
There's also a relay from the horn and the "air horn" switch on the dash. it's located on the side support next to the battery on mine. Check to make sure you get 12v to both the relay feed (dash button) and the relay input (horn button). Together they drive the solenoid. The wiring diagram doesn't show this real well.
It's not complicated, just tight working space. I removed the front end on mine for other reasons which gave good access to the system.
Good luck - the sound is awesome!