Lay on the floor and look up under the dash, there's usually a ton of room in there. Maybe you could find one of those AC units that mounts on the tranny hump, and gut it to stick it inside.
Be creative, it'll pay off.
As to the wiring, you can get a marine ignition switch (simple, only 2 connections) and put it in an inconspicous location to lock out the system. I had a small system in my wife's 84 regal once. The switch was mounted in the cig lighter hole and with the key removed was almost invisible in plain sight. With the key "armed" an indicator light would come on in the dash and a relay under the hood would re-route the horn wire to the nitrous solenoids. Simple, effective and stealthy. A bit hard to steer in corners under power though.
I've also heard of people fixing the foot switch for the High/Low beams to be momentary contact. This doesn't work as well with stick shifts.
If you want it out of sight but don't need it to bear close scrutiny you can mount it in the rear floor board and throw a bunch of trash over it. Or cut the bottom out of a tool box and set that over the top of it.
