Mercedes Benz repair: warm air with vent closed 89 560sl, mercedes benz 560sl, number search results

Question
With the vent/heater/air off there is warm air coming through vents on a 1989 560sl. Is this normal?

Answer
Best guess:
The mono valve has failed = default to max heat.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1989 Mercedes Benz 560SL
Part: Mono Valve Repair Kit
 Note:  Heater Valve
Electromagnetic heater valve.     
R3023-15174 Mono Valve Repair Kit Bosch IN STOCK $56.83  

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