Two common problems
1. The linkage will bind: to rectify, just take apart one at a time and grease
2. The amp is a fault. By far the most common but expensive. To rectify, replace control unit.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM