The space that you have to work with is 0.75 cubic feet.I'm actually looking to do something similar to you.
You probably should get shallow mount subwoofers to get the most out of the small space. Pioneer Premier TS-SW1001S4 subs would fit perfectly.
I suggest you make a box for your aftermarket sub. (you could probably pick up some medium density fiberboard from Lowes and make it yourself)
You're going to need an aftermarket amp. (JL Audio HD750/1) And also a line output converter; (AudioControl LC6i)
I got the box space specs by doing the following:
1. Rockfordfosgate.com
2. Highlight Support
3. Click on Woofer Box Wizard
4. Fill in the blanks.
GL with your project!
Modified by maximus108 at 10:54 AM 6/26/2009