The most logical answers involve mounting in the rear shelf firing into the compartment like sux2bu and juggy posted. The sub in the rear door isnt a bad idea, but it is limited by the amount of cone surface area that they provide for the car. Look at Parts Express and they have lots of 5, 6 and 7 in subs.

You could put a 10" or 12" flat sub in a flat enclosure and put it in the passenger legroom area

dont forget if you want tones that low and you want to put your drives in the door you need to build a pod in the door and seal it. i just cant see you getting low bass without an encloser