The outside, lower trim on the door, at the bottom of the window? Remove (or at least loosen) the side view mirror, and slide the trim forward then lift off. If your truck is an older model, the trim may have integral metal clips that you'll have to gently pry away from the door. That was actually a superior fastening method; the new style sucks and slides around.
Replacement pieces from LMC are junk. What's sad is then I got some from GM and they looked the same...
How you get to the mirror depends on what year. 1988-1994 remove the door panel. 1995-up you can do it just by carefully popping off the triangle trim piece; it has one push-pin fastener near the top/middle, and a tab going into the door panel. Careful not to drop any mirror nuts inside the door.
Richard