If you mount your amp in the center of the back wall, should be no trouble clearing the back seat. I thought about going this route, but since I have no kids and wanted more room for tools and for my dog, it made more sense for me to take the entire seat out. So my amp is hidden neatly along with my capacitor in the little compartment where your sub box is probably going.
I am going to get into building a different box this weekend or next, depends how lazy I am. When I'm done I will post up pics for everybody to see. Sometimes I wish I went with a smaller sub, easier to work with 8s or 10s. Now it's too late since I have a 12 that I like too much to part with. Glad I didn't get itchy enough to get a 15 again!