I've seen some designs for sub enclosures for the hatchback part of the car, but what I have in mind requires more than just the two subs in the trunk.

I've removed my back seats to find that there *should* be enough room there to fit two custom enclosures for my subs, which on one side of the box will have the 12" sub, and on the other will have two additional speakers. Has anything like this been done before, and if so, does anyone know the dimensions for it? It would be much appreciated for any feedback that I get.

 

Figure out what speakers you are going to use. Then how many cubic ft they need to operate properly. Get out your tape measure and get to calculating the dimension of box you will need and figure out how it will fit where you want them. For some ideas on boxes go to this site and see what other people are doing.
http://www.carstereo.com/installs/ne...w=1&maxrows=24