I'm not totally sure on this, but I think you're kinda screwed on the speaker installation behind the seats. There's very little wiggle room there, and I believe 12" will be too big. I think it's a 9" in there now.

However, several folks have successfully mounted them in the trunk area. The setup I like the best is user FritzAuf. He made a custom box for the trunk that sits on the sloped portion of the trunk facing the rear. His looks like it came with the car, and is a perfect fit. He also had enough room in the cabinet to stow some minor detailing gear, so it isn't rolling around the trunk.

Try dropping him a PM, he might be able to help you out.