Then in that case you could do two things...One raise the floor and sink 2 12"s of a single 10 what ever you want...You would have to raise the floor about 3 inchs, most of the time bring it level with the opening of the trunk...

Or if I were you I put put a single 12 in a sealed box and just push it up to the seats, firing towards the tail lights....You will still have lots of room and then you can mount your amp either on the back of the seats or on the box itself. A single 12 or 10 will get you a nice bump if you are just trying to round out the sound....