Front Seat Belt Retractor problems of the 1999 Buick Regal

We have had 6 incidents of the rear seat belt retractors locking up making them unusable. The failures have happened on the right and left sides. This leaves one of our two children using the center lap belt, until repairs can be made. Once both shoulder belts failed at the same time leaving us a seatbelt short. Gm claims this is the only Buick in central IL. With this problem and therefore it will cost aprx. 150. 00 to fix each time. I feel this leaves my children at risk. This problem needs addressed nation wide as I feel this must be a design problem not just bad luck.