Not sure what is rattling: trunk lid, or side panels.
But, a speaker system needs rigid enclosures, and that includes the car itself. or even in your home. the house and the car are part of the speakers.
for instance: Wood paneling in homes will rattle from low frequency sound...
Removing all cardboard or fabric paneling in the trunk, and spraying undercoating or polyurethane foam would help a lot to dampen and firm up the enclosure (the car :) )
or, put carpeting on the sides and floor, but they need to be securely attached to the sheet metal (contact cement might work).
Reinstalled the panels as well.
Dynamat
my experience in stereo work you get the deepest bass with the subs facing the open trunk area
The trunk of your car is the best place to mount a pair of subs. The trunk also acts like a sound chamber for the subs. Just make sure you seal the trunk good so it will not raddle when the bass hits.
The guy in the trunk is still alive!!!
well i dont really think that you can stop the trunk rattle not if the bass is high. You could turn down the volume.
Dynamat usually does the trick, also try removing the trunk liner and seeing if anything is vibrating noticeably inside it. http://www.dynamat.com/
balance it
The heat shield on top of the converter may have broken loose. it can be repaired. If the rattling is inside the converter it must be replaced.
well i took the seats out and mounted the entire box to my car lol, lot of work though and its complete custom job, honestly there isnt much you can do to stop that rattling if anything...These seats have a release in them to fold down so makig them tight so they dont rattle...Not to mention its a metal on metal hinge which causes even more vibration and a terribe sound in your system...ok anyway my suggestion and its pretty lame is just simply put the seat down if there isnt anyone in your back seat at the time and if there are a couple people back there the weight could stop the rattling significantly
Get a new car that's actually worth something.
The best way to stop the rear brakes on a Mazda 626 from rattling would be to replace them. New brakes can cost between $20 and upwards of $100 depending on the quality of the brakes.
I Like Them Facing With The Speaker Face. Otherwise You Defeat The Purpose