There's a ball bearing on a spring inside that does the detents. The switch is kind of a pain to open up though, I'd want another on hand before taking the one on the car now apart. Good new is they were used on lots of cars so finding a good used spare shouldn't be too bad.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.