Simply tap them when they pop up on the screen. Also you can plug in the rear seatbelts at all times.
This gets triggered when you put heavy stuff on one of the back seats, so it detects weight on the seats and tries to warn you that the occupants are not wearing seat belts.
Listen to this tip by Ron from Nashville in Ride the Lightning episode 210 at around 19:14.
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Photo by Bjørn Nyland aka TeslaBjorn.
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