Five Questions

How much air space is required for a 10 inch and 12 inch sub box?

Is there enough airspace under the rear seats of a CC for a 12 inch?

Does anyone have a plan or dimensions for a CC box under the rear seats?

What type of wood do I use?

Where do i get the carpet?

it depends on the specs of the sub. yes it can fit under there but it depends on specs. most use mdf at a hardware store for the box. do some searches here and there should be some guys who put dimensions.

Hey Skillz,
I've built and helped build boxes in the past. As rcantu just said get the MDF (particle board) from the hardware store. As far as air space goes. I was looking at some MTX subs and the 10" required .75 cubic ft and the 12" required 1.25 cubic feet. Each mfgr recommendeds the specs, but that will give you a really good estimate on what you have to work with. Carpet-you can pick up a roll at any auto store and i think wal-mart carries it also. If i had a table saw, i would start a project like this. However i may go with the pre-built MTX box. It has 2-10" subs and you can get it amplified or just buy your own amp. If you're gonna build a box and do this, you know you have to post some pics....lol
Take it easy,

Does it matter where the airspace is relative to the subwoofer, could i make a box that goes over the drive shaft of the truck making an L shaped box for more airspace?

I'm not an expert on how the shape of a box affects performance. But i think it would be ok to have the extra L over the drive shaft. I would think the extra volume would only help things.

I'm pretty sure it's the volume and not the shape that affects speaker performance. Specific shapes can probably have slightly different pressures in various locations, but I'm not sure it would be noticeable. I would think as long as you've got the volume you should be fine.