For anyone who is considering adding a subwoofer but doesn't want to lose what little cargo space there is, consider the Kicker shallow subwoofer with amp built in. It sounds great in my girlfriend's Cooper, and while it won't wake up the neighbors (that is a good thing), it is pretty accurate for such a small,shallow subwoofer, and blends well with the front speakers. It doesn't need a separate wiring kit and is so small that I was able to build a false floor to keep things looking clean and provide storage underneath for oil, jumper cables, and a first aid kit.

Before:



After:









The color looks a bit off in the photo but is pretty close in person. Rather than buy a big sheet of carpet, I simply used felt from a craft store, and a small piece of plywood from Lowe's.

I installed a handle so she can easily lift it to access items underneath. I will probably paint the under side black to make it look better. It has 4 inches of clearance for the sub, leaving around half inch of room, so I made a (somewhat crude) vent for the sub so the bass wouldn't be trapped. Probably didn't need this, as there are gaps around the edges, but I wanted to be sure.

The total cost for all materials (felt, wood, glue) was under $30 and for someone more skilled than me, would take less than an hour.

GREAT clean look. Good work.
Where did you pull power from? Where did you pick up the audio signal?

Here is my DIY rear seat delete. I got a larger Kicker L7 and I wanted to have it at an angle but also be built into the flooring. I also have the mono amp hidden under the false floor as well. The smaller amp for my other speakers is in the sidewall where the old Mini Amp was installed. (Sorry I know the image was dark, need to borrow the wifes camera at some point and get a better pic)

I didn't install the subwoofer, so I am not sure, but I would think you could wire it into the 12v power receptacle wiring in the rear. It is a speaker level input enclosure. There is an Alpine KTP-445u amp mounted under the dash for the front speakers (Infinity Kappa 60.9).