My previous post about the rotating shift knob in c900s:
Update:
My 1992 900 has a different design: the plastic ball is now metal, and is welded to the shift lever. The shift lever simply does not rotate independently of the metal plate that holds it onto the shifter housing. (The Bentley manual, btw, shows the old design)
The rotating part is actually just the emblem (gear map) in the shift knob- pry it out and re-glue it. Fixed it for me anyway.
I only discovered this after spending an embarassingly long time disassembling and re-assembling the entire shift lever assembly...
John