You must remove the rear door panel on a GMC Sierra if you are trying to replace the panel or the door handle. Since the Sierra's door panel is held in place with more than just retaining clips--unlike most vehicles--you must follow a specific procedure to prevent damaging the panel. If you've never done this job before, allow yourself at least half an hour to remove the panel. There are various plastic trim covers that can snap and break easily if you are not careful.
Pry off the small trim piece inside the grab handle on the door; use a flat-tip screwdriver to pry the piece off.
Remove the two 10 mm bolts behind the trim piece.
Pry off the trim piece on the other grab handle on the door. There is a notch in the bottom of the trim that you pry up on to remove this trim piece.
Pry off the trim piece behind the inner door handle.
Remove all the bolts underneath the removed trim pieces, and pull the grab handles off the door.
Pry off the small triangular-shaped trim piece near the door-mounted rear-view mirror.
Pry off the trim piece on top of the manual locking mechanism.
Wedge a small screwdriver into the center of the small square behind the trim piece, and very gently pry it apart. This will allow you to remove the trim piece surrounding the manual door lock mechanism.
Pull the door panel off the door. It is held in place to the frame with retaining clips. Simply pull the panel off the frame, and the clips will come along with it.
