[GOPHERIT]

Based on only seeing a single vertical trim panel, am I looking at the vertical plastic trim piece located between the moveable an fixed glass portion on the rear door ?

 

[GOPHERIT]

The best I can understand by words and pictures found on GM's IPC, is the following. The drivers side is called "F channel" with a a P/N 92233116 for $171. Passenger side is called "Pitch black gloss passenger side rear door stationary window channel" with a P?N 92233115 for $134 It looks like these items (besides the visual portion between the movable an fixed glass), also appears to include the channel guide for the trailing edge of the moveable glass. For sure you should visit a good GM parts department with these 2 P/N's and verify (at those prices) that they are what you are talking about. Can try to post R&R info from the service manual if actually needed.