European circuitry for the belt reminder is a bit different from the US approach. While the US system will switch off after a while, European won't
There is a load sensor under a passenger seat and if there is a short in it's wiring someplace the warning won't stop flashing until both belts are fastened. Try fastening both of the belts and see if it helps. Other than that you could look at the switches in the buckles, they might not work properly. Keep in mind that one set of the wires in the buckles is for the little red light that illuminates them in the dark. Unfortunately I don't remember the color scheme from the top of my mind, but it should be pretty obvious once you look at it. You could disable the system by pulling both of the bulbs, front and rear.