I installed an 8 inch pioneer shallow subwoofer in the rear deak and there is a lot of rattling back there. Is there a way to fix it or should i just get a box?

 

Take off the rear deck and put sound deadening. Or stuff foam tape in between the deck and the rear window, there's a small gap that allows the deck To vibrate and tap the window. You can also do that with the middle seat belt.