I use a Bazooka 100 watt sub - I put it in the well where the spare tire would go (yup, I'm living dangerously)...
I put carpet padding on the back floor (under the carpet) and around the sub to deaden transaxle/road noise, and to insure the sub didn't vibrate against the metal.
It works great and is way louder than I need or want. I like it in the tire well because it's completely out of sight.