Backseats are actually quite common for mounting amps, believe it or not. One reason is there is only so many places to mount, and that provides an option of having stuff on the cargo floor. Plus some people feel mounting amps on the subwoofer box is bad for the amp, due to vibration. I don't really have an opinion on that subject, as I don't consider myself an expert. My attitude is just mount elsewhere, just in case.
I didn't install my stuff, because I am quite cowardly when it comes to electricity, but my amp is installed in the wheel holder area, that dips below the cargo floor. I have technically never looked at the amp since having it installed. So its not visible at all. The trade off is I cant make EQ adjustments very easily (but I seldom EQ anymore).
Back when I was frequenting the car audio sites, (coincidently before joining headfi) Image Dynamics was very well praised for providing competitive products at lower-than-expected price brackets. They actually had a horn system for the tweeter in their product range, that mounted below the glove compartment and steering wheel. Some people thought it was amazing. I never heard it though, so I don't know.