Since the controls are a fragile item and known for breaking, do not force. Do as Dana suggests and review the numerous discussions regarding heater valves and controls via the search feature. I believe you will find that the cable that contols the valve is prone to freezing up also. If you know the cable has free movement, and the spray pennetrant does not work, you will need to get to the valve. These valves are also prone to leaking. Review the posts on this subject and you will know what you need to do.
Frank Koronkiewicz
Willowbrook, Illinois
1970 280SL Light Ivory