okay I found this on LXF its got directions on removing the hatch plastic cover with pics
Magnum Spoiler removal 101.
But I had the same problem and I found it was the speaker grill covers in the back on the sides they just snap off so they rattle if you remove them and put some felt on the lip it helps to totally stop the rattle. I had to turn up the tunes and get back there to figure out where it was coming from and thats how I found it. Good luck!