I have my 12" hifonics 1600wrms sub in my trunk WITH the mach460 sound system. I designed the box, and my dad helped me build it. Its somewhat of a bitch, but you take out the rear seats(pretty easy) then drop the mach460 amp rack(hard by yourself, its easier if you take off the carpet covering the speakers)
now it is NOT 14 INCHES! I built a box and found this out the hard way, there is a small lump in the chassis in the trunk which restricts you to about 13.25, 13 to be safe.
So I built another box with these exterior dimensions, 12.5 tall, 30in wide, 14 in deep. I had a total port length of about.. 24 in? But I have my sub facing up, ( yes I know the disadvantages) but it works and bumps pretty good and I can still have my mach460 up there. If you need help designing a box, shoot me a pm.