just about ANYTHING you do is gonna be custom...
if you have the 40/20/40 up front you could go w/
the JL Audio stealth box (replaces the center front
seat, reusing the original covering)...the stealth box
is hard as a rock...almost rendering the front center
seat unusable for anything other than a SHORT ride.
no one makes box for behind the rear seat that im aware of,
theres not alot of depth so youll have to be careful about
what sub speaker(s) you choose.
pioneer makes a FLAT shallow sub, and JL Audio just announced
some.
Im using a Kicker Solobaric (square) 10" L5 (shallow depth) model
and i built my own box.
your next question is gonna be amps...and its either mounting 'em
on the rear wall or under the rear seats (but youll have to remove
that lovely tray systems to get just about anything under there).
heres some links to my set up -->
donrumsey/truckaudio - Photobucket - Video and Image Hosting