if size is a concern i would go with a single 10" sub. just stick with a well known and respected brand. (rockford-mtx-jlaudio just to name a few). depending on the type of sound you are after will determine the type of box you should choose. band pass is loud and mainly for SPL - not very versitile and must have a larger box. ported is also louder(?) but bass can be a little loose although the box can be smaller than a band pass. sealed would be the smallest box possible - and also the most accurate of the three for bass reproduction - very efficient but will take more power to bee as loud(?) as the other two boxes. i would strongly reccomend the sealed box and a good sub. just make sure the interior box dims are to the sub manufacturers specs. power it up with a quality and powerful amp and rock on!
if you get a quality sub/box/amp combo you will hear the bass inside of the car. some people like to put the rear seats down... but i think that looks crappy. if you face the sub towards a rear corner of the trunk, the bass will magnify and bounce forward. whatever you decide, play around with placement to achieve maximum results.
My Mirage does not have a folding seat..(or split/fold for that matter...)I am running Infinity 6.1s in the rear deck for fill...won't the bass output of the sub box interfere with the cone movement of the Infinity's?.
should not interfere with the 6.1's. just make sure you go to a good shop so you can audition what you would like to buy and maybe the shop can make some reccomendations for you... dont let them sell you something you dont want!