[Msquared]

Sorry I'm just now responding. The Stealthbox seems like an easy-button solution. I have one alternative suggestion that will maximize trunk space, just an idea to ponder:

Instead of a separate sub, you could drastically improve the upper- and lower-frequency extension and sound quality of the door and dash speakers. Hear me out (pun intended). I'd approach this by replacing the dash speakers with Audiofrog GS25 single-driver units, which have a much better treble range but are also designed to be crossed over to the door speakers as low as 200Hz. Then I'd replace the door speakers with their GS8ND woofers. These are shallow-mount 8" drivers meant for a free-air (infinite baffle) installation like a door or rear deck. They play down to 30Hz. The only catch here is that you want at least 75wpc to use them, and preferably you use DSP that can actively cross the two drivers over. If you do that and set the EQ and time delays properly, you'll get vastly improved sound quality with much higher highs and much lower lows...not to mention much more volume if you want it. And you get to keep all your trunk space! This will surely cost more than adding a trunk sub to the otherwise stock system, but it will be a big difference in sound quality.