W201 has no monovalve like the W126.
W201's heater function is done by a vacuum controlled valve.
Photo of valve here, next to blower box thing.
Mercedes-Benz Forum - View Single Post - 190E 2.6 Heat coming from both vents in the cabin
Check to make sure the HVAC system is getting vacuum. I recall accidentally disconnecting mine near the brake booster and getting heat.
Could be vacuum leak too.
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