Had the same problem in the '73 220D. The issue was dirty contacts at the blower fuse on the firewall. That fuse is not located in the main fuse block, but has its own box in the engine compartment. In my car it's located high on the RH side firewall. Could be in a different spot on the 300D. Worth checking out.

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- Greg -
1973 220D, The Prodigal Benz
1974 240D