Typically I'd say the switch itself has failed.. have you pulled the steering wheel to take a look? There is a tripping mechanism if I remember correctly...it's just been a while since I took a look @ one.

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
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Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta