a lot of 12" subs only require 1ft of cubic space. Go with Alpine Type-R's. If you get the 4 ohm subs go with two Alpine MRD-M300 amps, if you get the 2 ohm subs go with a MRD-M500.
BTW, most people have their subs in the trunk and there is a wall there called seats that separate it from the cab and they can still hear the bass quite well, me included.
