201 car has an on/off vacuum switched heater valve. The quick fix is to locate it in the cowl area and run a vacuum line to it. If you still have heat and the valve moves with vacuum then the gate in the valve is bad. The quick fix is a cork in the hose.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician