Hey Folks -
I picked up my 2010 Mazda3s 5 door (velocity red, 6 speed, Bose/sunroof) Saturday. I'm a happy camper, except for that horn! Really anemic! Back in the day I added little Italian air horns, 2 trumpets, about 5 and 8 inches, a little compressor about the size of a soup can, to a '69 Datsun 510 and a 1980 Dodge Colt FWD. The Datsun install was a snap, the Colt more difficult. With both of those I left the stock horns (such as they were) in place and used the stock wiring to run the little compressor, mounting it and the horns wherever I could find a place. I haven't had time to crawl over and under the engine bay in the Mazda. I bet it won't be an easy process.
Other than that, its a sweet ride. Reasonable grunt, very low wind noise, the Yokohama tires are a bit loud. Love the direct steering and the "right now" handling.