I originally made custom boxes for myself to fit under the rear seats - they sounded terrible! Not enough room for the 10" subs to "work" just right. I'm not positive but the '03 and '04 Avy has a different rear seat set up than the older Avy's. It will only accomadate one box on the driver's side which will allow you to put your amp behind the passenger side. This is an inexpensive way to get some bass without dropping some bills for a mid gate replacement. The boxes I made were 11W X 11L X 6H that tapered down to the bottom being 10W X 10L. I just recieved a box that I bought off e-bay item#5700404039 and it sounds GREAT. Box needed to be cut in the back corner a little so it wouold pass by a bolt for the seat but the cut is not visible after the box sits down where it should.