Mike,
The two side pieces need to be removed first. They are held on with clips that push in. There may be a clip on the corner piece that has a threaded stud with a nut. Not recalling for sure though. The piece that goes across the back is held on with threaded clips with nuts. You will need to get into the trunk to get to those. There are holes in the inner panel to access the nuts.
The side pieces are kind of a pain to get the push in clips to come out.
Loren