I think it is just a switch on the pedal, like all my other cars, though I don't recall ever looking for it on my W123 cars. There is generally an adjustment. It should sense the pedal motion without affecting it, i.e. pedal shouldn't bottom-out on the switch. Good it is only powered in "run". In many cars, it is always powered, meaning the battery runs down overnight if the brake lights stay on.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans