Mercedes designs the climate control failures to default to the defrost on. If you get any air throught the defrost when it should not be, it means that the servo for the defrost vent has failed and is stuck in the default mode (Defrost on all the time).

Try turning on the heat and see if it comes through the defroster as well. If the defrost will not go off under any condition it is most likely the defrost vent servo.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si