I want to install an air horn in my 2007 GMC Sierra?

[clark]

I want to be able to use both the air horn and the original truck horn separately by flipping a switch but I am stuck trying to figure out the wiring. Can someone give me a walk through or some kind of explanation to how it could be done.
I know that it would be easier but I want to be able to use my original horn button to be able to quickly honk the air horn

 

[4_3_beast]

why does everyone make statements and put a question mark after it? it's almost like they aren't sure they actually want to do what they are stating they want to do...hmmmm....