My guess is the dome lamp module is simply bad. It has no connection to the seatbelt switches, just the door switches, constant, and switched power. If the dome light and entry lights are working properly the problem likely is the dome lamp.

-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.