It’s been a while but... clockwise from top are defrost, bi-level, heat/footwell and dash vent. It’s practically continuous variation between any two marked positions. The dial controls a pair of Bowden cables in hard plastic sheaths so the cables are able to push and pull. IIRC the right dial is fully mechanical and independent of other settings. If it’s stuck, I’d say it’s most likely one of the flaps it actuates is stuck. The cables aren't very long. You might be able to see what’s stuck by pulling the control panel.

Sixto
98 E320s sedan and wagon
02 C320 wagon