Rear Dash Rattles
 

I have a 98 Seville SLS and it has the factory non-Bose woofer in the back dash. I have replaced the amplifier and now there is a terrible rattle. It is not the woofer I removed it and tested it (all the speakers work perfectly). I also removed all the carpet and liner from the trunk, including the spare. and have located it to the back dash its self, it is a plastic deal with the sub mounted in the middle. The light mount was rattling a little bit so I pulled it down too, but the big one is still there. What can I do short of removing it and building a new one? I could put a sub in a box in the trunk, but the dash STILL rattles (tested with a sub being powered by another car).
OH and someone had the back seat out before me, I think there are washers missing from where the seat belts and the bottom brackets for the seats attach. The only thing there is the seat belt mount(s), the bolts, and the brackets for the seats. How do I find out?

Rattles suck! You will need someone to help you isolate source. Once you find it then how to insolate it or attach it so it doesn't rattle. They did use washers mounting seat or seat belts. Washer would be made to nut or screw