You will have a fuse and a relay in the electric horn power supply.
The horn button grounds the relay to activate it , the relay feeds power from the fuse to the electric horn.
The dash switch for the air horn , adds the air supply solenoid to the power feed from the relay.
Fuse and relay in the Freightliner power centers , air solenoid ( on my chassis ) the air solenoid is attached to the customer manifold , just to the drivers side of the gen set .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.