You have to replace the pump. It is located behind the rear panel of the trunk. It is in the top left corner as if you are looking at the back seat from the rear of the car. We do sell these in the partshop.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM