Take apart the seatbelt buzzer relay and you will find one of the traces inside is blown up. My car had the same problem when I got it. It was easily fixed by soldering in a wire in place of the burnt up trace. The buzzer relay is mounted up in the dash by the steering column, on the same bracket as the cruise control unit.

-Jason

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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.