Usually you only get a seatbelt warning light on the dash when someone has not fastened their seatbelt. For instance when no one is sitting in the seat no warning appears. I don't know the set-up on your exact model but it could be faulty seat sensors rather than the belt mechanism. If that is the case then I would suspect a wiring problem.
Have you tried getting someone to sit on the other seats to see what happens?
I'm only getting the 2 rear seatbelts fastened message on dashboard. Tried them all separately, when you put the passenger side one in it changes to 3 then back to 2 when it's unclicked. The other 2 don't change the message so these two are the faulty ones. If I take the seats completely out then the message is cleared from the dash.