Driver's seat belt not fastened/unfastened warning light is on/flashing

Symptoms

["The warning light comes on each time the ignition is switched on and remains on until the driver fastens his seat belt.", "Above 12 mph (20 km/h), the warning light flashes for two minutes accompanied by an increasingly audible signal and a message on the multifunction display.", "Once these two minutes have elapsed, the warning light remains lit until the driver’s seat belt is fastened."]