i dont think theres more than a few inches of depth under the seats. youd have to more than likely raise the seat, make your own box, use a shitty ass sub(thats very shallow), and itll sound like ass.
you want some thump thats cheap but sounds nice? i was bored and went to the audio place by me, they had 1 10" Punch HE2 in its own ported box for 150 bucks, and it was hooked to a 400w kenwood amp(which was 100 bucks), and sounded super clean. 2 of those 10's and a 800w kenwood amp would sound very nice, and not break the bank, and still fit in the trunk and leave you lots of room. i say 10's, cause 12's are very hard to get into a stangs trunk, you almost have to make the box in the trunk, trust me. check out my cardomain if you want to know more(link in sig)