If you mean the interior trim panel, it shouldn't be necessary to remove that. There is a trim piece that goes over the part of the taillight that is covered by the tailgate when it is closed.
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Open the tailgate. Remove the two bolt covers on the rear pillar by prying on the bottom edge with a flat-tip screwdriver. Remove the bolts with a Phillips-head screwdriver, and remove the light assembly from the rear pillar.