I know they didn't "add" the lock at some point and that it wasn't available on the early models...because I had an 89 with the lock, and I am pretty sure every other early model trackick I have has the lock (90s, 91s, etc....I don't think I currently have an 89). Now, it may have been an option to either add or delete it on some models.
Of course a TT rear door won't fit a convertible....I was talking about convertible tailgates.