Unfortunately my car was missing the piece of trim I circled in red in this photo. I managed to source that trim piece online, but I'm not sure how it's installed. Do I have to remove the window glass? There are what look like screw holes on the molding, but I don't know if that screwed in originally or if there was some type of clip. The second photo shows my car with the missing molding.

 

That's a straight Buick. I'd start by removing that door panel. With the door panel removed you should be able to see how that trim is attached to the door.

 

They attach like any other sealing strip. From the inside of the top of the door panel. Little screws is all. As stated, remove the inner door panel, then you may need to remove the bottom window stop and lower the window a bit further down to get to all the screws.