Also, you may want to consider putting your battery in a location other than all the way in the back.
Without a long explanation of the physics involved, the best spot for your battery is as low and as centered in your car as possible. On my 1st Gen CRX, I put the battery on the floor directly behind the passenger's seat.
I use a gel cel battery (a generic brand, do not duped into spending too much on a "name" brand). I use a riding lawn mower battery for the low weight and small size.
If you plan on annoying your neighbors with your poor taste in music with a monster system, buy a couple of the largest batteries you can afford.