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Defroster button, location / task / function - GF83.40-P-2139GI

MODEL 163 as of 01.09.01 with CODE (580a) Automatic air conditioning 

Fig 1: Identifying Blower Switch And Defroster Button
G04608956DEFROSTINGDEFROSTINGCourtesy of MERCEDES-BENZ USA
  Defroster button, location The defroster button (DEFROSTING ) is located in the center in the upper half of the blower switch (1) on the AAC push-button control module (N22).  
  Defroster button, purpose The defroster button (DEFROSTING ) starts the defroster function.  
  Defroster button, function   GF83.40-P-2139-02GI