If the car is the 190e in your signature then it has a vacuum operated heater valve in the cowl area. See if the vacuum is on it at all times. This is often rigged to stop the heater from messing with the AC. If it isn't rigged, then take the vacuum line off the valve and see if the arm moves. If it does and the hoses aren't rigged then you have a bad valve. They are dirt cheap so don't go to heroics to avoid replacing it.

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