You might try jumpering the appropriate terminals in the small junction box on the passenger fenderwell. If that causes the starter to operate, it pretty much rules out the starter or solenoid, and tends to point to the key switch and/or the NSS. Believe the appropriate terminals are the two outer ones, but it has been covered here on the forum before.

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