Jus throw a lot of dampening back there. Strip out the carpet and cover all the bare metal, plus the inside of your hatch door. It will cut down in the rattling and give you way cleaner bass inside of the car. Im going to be running a 12'' SI MAG at about 1000wrms so i am gonna need all the dampling i can get. Check out RAAMMAT.
I had a 2001 zx5 and to keep it from rattling i used hood pins!!! Seriously.. Through the hatch one on each side and i replaced the rubber seal in the back with a thicker one so i had to realy suck down the hatch to lock the pins..