You could use one of the spare-tire subs that came in a few of the newer cars out there. They're flat and fit right on top of your spare tire. Depending on the size you choose, it would give you a nice bit of bass without having to modify anything in the back with 6x9's. Instead you could put a few 6 1/2's in the back which would be easier to install and put a couple of 4's in the front kick panels. Between the four speakers and the sub, should sound pretty good.