Check the heater valve - do a search here either on monovalve or duovalve (I don't know which one you have). With the symptoms you describe there is a strong possibility this valve is bad. There should not be any heat coming out of the vents even if the flap is bad, since the valve shuts off coolant flow to the heater core when activated. I know on the W210 chassis the first sign of a failing valve is hot air out both side vents (versus hot air on just one side which is a sign of refrigeration problems). Like I said, you can try clamping the heater hose around the valve to see if this stops the hot air from blowing - if it does, you have narrowed down the cause of it.

Believe me, you'll be in deep if you start tearing that dash apart. I would do that as a last resort...

Do a search is all I can tell you.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280