"Starter lock switch" is Mercedes-speak for "neutral safety switch" and is probably a direct translation of the long German word for the switch.

You "lock the starter" to prevent it from operating with the switch when the transmission is in a driving gear (Drive, Slope, Low or Reverse).

The switch also has an internal relay to power the backup lights when you shift to Reverse.

If you can't start the car in Neutral or Park, or if your backup lights don't work, then it is time to replace the switch.
The transmission will still function even if it is unplugged.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022