I would echo the solenoid possibility as well, and from a long term 220D driver's experience. The first time it happened we were in Twodot, Montana of all places. For whatever reason, I decided to whack the solenoid with something fairly heavy, and it worked. It happened several times on the trip and the whack solution worked every time. Took the starter off once we got home and cleaned it up inside and inside the solenoid. This seemed to fix it for sometime, but later on it re-appeared. That time it seemed like the contact areas where all the wires connected were corroded and thorough cleaning helped, but every once in a while I had to short the poles with a long copper rod, essentially replacing the "ign switch". Never did replace the starter in the 20 years we drove it.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150