alright im not gonna lie it seems crazy but why not.
I have an aftermarket alarm on my car and i am gonna replace the horn for the alarm with a cheap airhorn.
i want to know how i could run the + and - wires from the alarm horn to a relay and then to a momentary on off switch.
I want to be able to lock/unlock the car and then the airhorn will sound and then honk the airhorn separate from the car factory horn (with the switch).

any ideas on how this would work would help

 

I am pretty much guessing here, but I would do this: Check to see if it is 12v coming from the horn at the steering wheel (12v at the button), if it is then cut the wire farther down the column, and wire it to a 2-position switch (from the horn on the center pin, to the original horn on one of the sides). then run a wire from the third pin on the switch, through the firewall, to the air-horn + pin. Wire the air-horn - pin to any metal surface to ground it. If there is just a + and - on the old car alarm, just run the 2 wires to the air-horn. If there isn't 12v qt the horn, then I would run 2 wires to the firewall and use a regular momentary push button, and wire it right to the battery. Hope that helps a little, that's how I'd do it.