I have a Nissan Skyline 350GT 2007 Sport and wanted to know if the front passenger is not wearing a seatbelt should the warning light come on? At the moment it only comes on when the driver is not wearing a seatbelt. Or maybe it's a problem with my sensors?
Most seat belts are "checked" via two sensors: one in the seat (to check for pressure ... usually around 60-80lbs to activate); one in the click mechanism for the seat belt (to see if it's engaged). I'd suggest one or both of these sensors are bad. If the seat sensor never closes, it doesn't worry about whether the seat belt is buckled. If it closes correctly, but the buckle sensor is closed all the time, it would think the seat belt is being properly engaged.
You could try testing these by disconnecting the wiring. Jumping the two wires in the seat would show as though it is closed. Same with the buckle sensor. You could also run a continuity check on both to see if they are working correctly.