Just happened across this thread and thought I would post for posterity. I got the Kenwood one last year and it just barely fits under the front passenger seat in my wagon. It basically makes the seat non-adjustable though. I don't care, but some might. Also, it would not fit with the carpet, so I ended up using VHB tape to secure it directly to the floor and then you can deal with the carpet however you want (cut it to fit in my case).

Otherwise it was very easy to do. It definitely helps the sound, but even with the nicest speakers I could find for the back I just couldn't take it, so now it has 6x9s in the ceiling (Concours be damned). The Kenwood "sub" definitely has some punch, but there are two songs I always use to test a sub (Ludacris - Blueberry Yum Yum and Gorillaz - Clint Eastwood) and they don't even register on it. So now I'm trying to figure out something that will fit in the floor where the vomit seats used to be. You sedan guys have it easy!

-Rog