WHILE SLOWING TO TURN, VEHICLE WAS STRUCK FROM BEHIND, OTHER VEHICLE WAS MOVING FROM 35-40 MPH. AT IMPACT, DRIVER SEATBELT CAME UNLATCHED, CAUSING DRIVER TO SLIDE FORWARD, HITTING KNEE ON DASHBOARD HARD ENOUGH TO DISLODGE RADIO. AFTER MAIN REPAIRS, VEHICLE WAS TAKEN TO MARTIN FORD SALES, UNION GROVE, WI. THEIR INSPECTION FOUND A SMALL PIECE OF PLASTIC OR CELLOPHANE, NO MORE THAN AN 1/8" THICK AND 1/2" LONG. AFTER BEING UNABLE TO DUPLICATE THE PROBLEM WITH PANIC STOPS, THEY SAID THE DEBRIS 'MIGHT HAVE CAUSED THE CONCERN'. THEY WOULD NOT REPLACE THE SEATBELT OR LOOK INTO IT FURTHER. WE DO NOT FEEL THAT THE SMALL PIECE OF CELLOPHANE COULD HAVE CAUSED THE PROBLEM, NOR DO WE FEEL THAT PANIC STOPS DUPLICATE A REAR-END COLLISON.