I've already applied Damplifier underneath my rear deck, and the trunk (the part that opens only). It has already helped with rattles and my bass is now somehow much more pronounced and not as drone-y. I have two sheets left, can someone recommend a good place to place it? Trunk floor maybe?

 

hold off and wait till you hear new rattles and apply them to the places that make noise.

 

yep. that or buy a bit more and deaden your doors.

 

The only *real* rattle right now is coming from my rear passenger side seat belt, it keeps hitting on the seat belt holder thing (to prevent it from sliding down).

Also, I know a few 5th genners say rattles also come from the enclosed seat belt clips on the rear deck, how should I do those? Cut a small piece and stuff it in the cap?