Thinking about it.... to have a warning for the passenger seatbelt, there would have to be some way of telling the system that there was someone in the passenger seat. Otherwise you'd have to have the belt clipped in permanently. On early cars, that could be done by the OCS (Occupant Classifcation Sensor) fitted in the passenger seat. Later cars didn't have the OCS, so perhaps they didn't have the passenger seatbelt warning either.
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