I have seen these switches fail altogether or loose a ground. It can be checked and repaired by a tech easily.

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Benzmac:
Donnie Drummonds
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM