Talking about the weatherstrip piece that runs along the lower part of the window opening, on the outside? Pretty easy on 1995-up; easy on earlier models too. It's held in place by the clamping force of the side view mirror, a little tab at the rear that hooks under the door sheetmetal, and tabs along the length of it into the door edge.
Start with rolling down the window. Next up...what makes it easier on 1995-up is you have that removable triangle of interior trim above your door panel behind the nuts holding the mirror on - held on by a single panel fastener and also hooks into the top of the panel. For 1994-older you'd have to remove the door panel as that large plastic piece is bonded to the panel. Anyway get that out of the way, loosen but no need to remove the 3 nuts holding the mirror base to the door. Slide the weatherstrip forward towards the mirror and the tab at the rear will clear the door sheetmetal. Then lift it straight up off the door edge. Usually the tabs are weak enough it will lift right off. Older models with original weatherstrip have metal tabs that will require gentle prying with a pick or small screwdriver. New piece pushes straight down onto door edge then slide it rearwards to hook in the tab, then tighten the mirror to clamp it down.
DO NOT buy these new from the aftermarket. I got some from LMC for around $20/ea and they seemed to fit nice and tight when brand new, but within a week or so the tabs along the edge got loose and the weatherstrip would slide forward on its own and start lifting off the door in the wind. Really aggravating. I went back and bought them from GM, around $35/ea, look the same but I'd hope quality is better. Didn't have a chance to swap them before I sold that truck, nor do I have the part #'s handy since the receipts went with the truck.
Richard
