I ran across a cool subwoofer for guys who may be interested. It's GM#25722342, and was made by BOSE for the 98-04 Cadillac Seville/Deville. It is for the rear package shelf on those cars. It measures a very thin 2-3/4" total depth-magnet and all! It's 12" round, and measures 14" edge to edge across the mounting holes. It has a metal cone center, and pretty heavy material for the main body as well as a metal spider frame for support. It only weighs 3 pounds too.

I was amazed at how thin it is for the size. Good sound quality, and it doesn't need a sub box like some. GM prices it at about $136 ea, could be had for less too. Seen some on ebay, but I always shy away from used electronics.

http://i168.photobucket.com/albums/u165/Bowtie-Bry/BOSE.jpg

http://i168.photobucket.com/albums/u165/Bowtie-Bry/tb_9801EFSTBE.gif

This might not fit on the package tray, but it would be an easy trunk mount.