My one only has a seatbelt warning for the driver's seat. All 4 doors will indicate when open, with a graphic to tell you which one. Does yours show seatbelt for all 4 seats?
On my wife's car (not a Jazz) there is a connector under the back seats, and if you disconnect this, then the rear seatbelt sign does not show if there is someone sitting there unbuckled. It is a requirement that all passengers have their seat belts done up and if you get stopped in a roadblock, then the driver is in trouble of the policeman sees this.