Hit the upper clip in the front pillar trim (A) with a rubber mallet. The clip is located under the point where the triangle mark (B) on the edge of the trim indicates. Hitting the clip slides the projections (C) on the pin (D) and pushes it into the grommet (E) and against the body (F).
NOTE: The grommet must be replaced with a new one when the front pillar trim is reinstalled.