Right ok so I was fixing my rear speaker covers from an improper speaker cover reinstall but there was still a rattle... I went hunting. from 9:30PM in my garage till 10PM I hunted in the darkness.

Then I finally found the damned thing! It is the trunk light!!!! The trunk light bulb cover rattles like mad!

Solution: Cut a tiny piece of foam or similar and place around edges w/o obstructing the light (this step is optional). Then use clear tape and tape that mother down like there's no tomorow! Actually.. since it is upside down. Tape that mother up like there's no tomorow!

 

Well my tape wouldn't hold. I dunno about yours. So I just removed the light cover, squeezed it lengthwise and it poped right out. Hopefully this solves my rear rattleing forever.

 

Silicone