That 2nd switch is in my '74, was in a '76 I parted out too, and is probably in the '73. Now I'm really curious . . .

Dug into the wiring diagrams. It shows two switches, RH and LH for the "courtesy light", and a separate single switch for the warning buzzer. The circuits for each system are independent. The diagram notes that the buzzer's switch is the upper switch in the LH door opening (US market). Guess yours (OP's) got swapped between positions at some point.

Perhaps Benz added the buzzer circuit to an already established wiring scheme without a complete redesign of that part of the system.

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