The have four clips thingys that hold them to the rear deck. Best way to remove them is to go into the truck and you'll see them. Using some pliars you can get them out, just take you time doing so because they are crapy just like everything else in these dam cars.
Just get your hands under them and pull each corner out. You can also access the clips from inside the trunk but you might have to jump in the trunk for that. They should pull right out with some force.
I used the 4 corner pull method. Only thing is sometime the clips stay in the holes and not on the grills. 96SS is right about having to get in the trunk for that one. Might want to get in there w some pliers to close the little "grabber" on them so you can push them out.