Bodykit Rear Bumpers
https://celicahobby.com/gal3/inst_rearbumper When installing any complete aftermarket rear bumper replacement, you will need to remove all pices that were currently attached to your factory stock rear bumper... this includes the foam core, the plastic clips and both of the inside plastic fender walls.
Any bodyshop who has history at installing body kits should know this from previous installations (same aplies for Honda, Mitsubishi, WRX...etc)
As with MOST body kit pieces, this should be installed by a professional - but I've posted these photos for anyone who is curious as to how/where the actual install points are located - these photos should answer most of your questions