I would suggest that you build a box out of 18mm Medium Desnity Fiberboad and brace the panels so that all you get when you rap them with your knuckles is a dead sound not any resonace like you would get if you hit a drum skin. build the biggest box you can get away with. a 12" usually need about 2 cu. ft. + to breathe but like was mentioned earlier if the box is too small and the bass too lumpy one can tune this to some extent by using an aperiodic vent.
ofcourse if you are inclined you can follow a more extreme method of building a box. see post 9 on this thread. It is not very extreme but will suffice for your car box.
http://www.team-bhp.com/forum/showthread.php?t=6491 (Thiele-Small parameters)
I have seen some very extreme boxes using cross ply carbon fiber like this one
http://www.wilson-benesch.com/reviews.html
or
http://www.6moons.com/audioreviews/danadigital/108.html