Wiring 2 horns off switch

There's a black wire in your steering column that controls the horn.  When you push the horn button, this wire grounds, and your horn honks.  To test this wire, take a simple test light, and probe the wire.  Touch the other end to the rim on your cigarette lighter.  The horn should honk if this is the correct wire.  What you need to do is wire up a relay to use this black wire as your trigger.  You can wire everything inside the car (except the air horn, of course)- no need to go to the battery for power.

Your red wire at the ignition harness is your constant (verify with a meter).  Run a fuse holder from this wire to terminals 86 and 30 on the relay.  Connect terminal 85 to the black wire at the steering column that triggers your horn.  Now, run a wire from terminal 87 on your relay out to your air horns, and ground the other terminal to complete the circuit. 

If you want to be able to switch from your factory horn to the factory horn with the air horn, cut the wire that goes to terminal 85 on your relay, and put in a simple toggle switch.  When the switch is off, only your factory horn will sound.  When it's on, both the factory, and air horn will sound.