Mercedes-Benz 300D 2.5 1990 Owner's Manual page 26

0 Defrost
The maximum amount of heated air is directed to the
windshield and adjustable air outlets (8) independent of
the positions of the temperature selector wheel and fan
control buttons.
0 Off
The air supply and heat to the car interior is shut off.
While driving, use this setting only temporarily.
Defrost
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