Had a similar experience with my car. Turned out to be starter solenoid. You need to determine if the small terminal on the solenoid it getting voltage when the ignition switch is turned to the "Start" position. If not, then look at circuits from the ignition switch through to the solenoid. If voltage is there, it is either the solenoid or the starter itself. I understand that the solenoid grounds through the starter, so both units would have to be checked.

Hope this helps. Good luck.

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Ted
1979 240D
165,000 miles