Add bass to your car? Your rear deck is gonna rattle like a SOB. Props to the wife for this one: grab a cheap noodle at any dollar store. Cut it into sections and jam it up into the holes in the metal of the rear deck from the trunk side. Add some packing peanuts to the child lock latches while you are at it. Tape or glue would also work but come on!
I know the expensive and time consuming method is to disassemble the rear deck and custom cut some not so cheap Dynamat. But the noodle was a big win for me!