AT STOP LIGHT. HIT IN REAR BY CAR GOING ATLEAST 45 MPH. NECK WHIP-LASH DESPITE 3 POINT SEATBELT AND ADDED THICKNESS HEADREST AS SEATBELT COLLAPSED BACKWARD TO HORIZONTAL POSITION. UNABLE TO MOVE SEAT FULLY UPRIGHT (80%) AFTER THAT. CAR PUSHED ABOUT 20 FT. TOW HITCHED ATTACHED TO FRAME REDUCED FURTHER BACK DAMAGE. METAL PARTICLES FOUND IN DIFFERENTIAL - PROBABLY FORM IMPACT MOVING CAR IN STOPPED POSITION.