... umm you could get all 4 of the 10s or 12s with the crescendo amp, wiring and a XS 3400 for well under 2k.. Im guessing 1500.. which is right in the ball park he wanted to be in .. its his call on what he wants to do with the rear seats..if you're gonna do 4 subs you're gonna need real power, which means at least an extra battery and 0 gauge which isn't cheap. I'd just suggest a quality 12 and only remove one seat. bobby32 on here is running a PSI reconed Audiopulse Axis 12 and doing high 140's.
Here's a box I put in a full sized extended cab Dodge (approx the same space):
Box was average sized for dual 15's, 8 cubes gross. I never tried putting the back seat back over the enclosure, but it might have fit.
If you mean the one I posted, it was destroyed in a car accident before I finished installing the stereo. Yes, it was heavy as fuck. Even bolted down, in the accident the box broke free from the false floor and smashed into the front seat, breaking it loose from the floor and pushing the seat into the dash. And that was before I even added the 150 lbs of subwoofer to the box. And people wonder why I harp on them to bolt their boxes down...that 8 cube box is nice,heavy as FK I would imagine ,Id tint the windows to wart off thieves again