What I did for switching was put a cheap little switch from Kragen/PepBoys inline on the power cable for the airhorn amp. I usually leave it on, since it doesn't take much current on standby. When I want to just honk the regular horn, I just turn off that switch, honk, then turn it back on. It makes it a lot easier when somebody pulls out in front of me or whatever; I don't have to go looking for an airhorn button, I just push the steering wheel and they go deaf.
I don't use the PA on it, but I figured I might as well have it since it was only like $20 more. Never know when it might be useful.
I put the amp right under my driver's seat, and the speaker sits right behind the logo on the grill, attached to the small vertical post that's right behind it. It isn't exactly perfect, but it works.