In my case it was the starter switch. My original starter suddenly turned on while i was driving. I suspected the starter bendix since it was over 30 yrs old. It didnt just happen once. The last time it happened, the ring gear ate the bendix gear which left me stranded at work. When I installed another starter, it happened again. So, for now, i rigged up a push button switch. I still use the key for the vacuum and other accessories. All is good now.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles