WHILE VEHICLE WAS STOPPED IT WAS REARENDED. HAD CHILD IN EXTENDED CAB BENCH SEAT. THE SEAT COLLAPSED BACKWARDS & CHILD'S HEAD HIT SIDE OF CAB. THE FRONT SEATS COLLAPSED BACKWARDS ONTO CHILD SEATED IN REAR. CHILD SUSTAINED SERIOUS INJURIES FROM THE SEAT. BOTH SEATS FELL BACKWARDS UPON IMPACT. VEHICLE WAS TOTALED. *AK