Just removed the trunk lock latch cover on my 850 after numerous failed attempts to get it off over the last 12 years.
I had to succeed this time as the trunk wiring harness had failed at the left hinge, with no tail or license plate lights.
Too simple yet too hard!!
So, all you need to do hold the cover by the long side and pull it hard to the left while pushing on the other side as you wiggle it up and over the left side of the keeper on the nut.
Once the left side of the cover slips from underneath the keeper just push the cover to the right and it slips off.
I anticipate, the installation will require the reverse effort.
That is, place the cover over the latch and set the cover under/into one side of the latch bolt keeper and then pull/push the cover to the side that needs to be locked under the latch.