turn your radio up, then go to the back by the trunk and listen to where it rattles the most. then apply pressure to certain areas and listen for the rattle to die out to find out where exactly the rattle comes from.

 

i dynomatted my trunk also. I dynomatted the top part of the trunk(away from the keyhole). I also dynomatted the sides of the car in the trunk area, and both license plates. The guy who i bought it from said i need to buy some more to cover under the back speakers, the other side of the trunk (part where the keyholes at), and under the spare tire. The guy before didnt put it on right so my car had all kinds of rattle. But he said he had to use the roller again and it only now has a low rattle. It's still bad though and needs to be taken care of...i need some $$$$$$