WE WERE IN AN ACCIDENT WITH OUR 2018 DODGE GRAND CARAVAN WHERE A PERSON REAR ENDED US GOING AT LEAST 75 MPH WHILE I WAS GOING A MAXIMUM OF 20 MPH. WE WERE IN A CITY MAINTAINED ROAD, SILVERBELL RD AT THE INTERSECTION OF CORTARO RD. MY SISTER IN LAW WAS IN THE VERY BACK 3RD ROW PASSENGER SIDE CHAIR WHEN IT DISLODGED FROM THE METAL HOOKS ON IMPACT. THE SEAT FLUNG AROUND WITH HER IN IT AND THE HEAD REST HIT SO HARD AGAINST THE BACK WINDOW/DOOR THAT IT IS NOW PERMANENTLY BEND FORWARD. WE CANNOT GET THE SEAT TO STAY UP ANYMORE OR CLOSE DOWN INTO THE GROUND. THE CAR IS NOW TOTALLED. IN THE IMPACT, SHE SUSTAINED 2 BIG BUMPS ON THE TOP OF HER HEAD & LEFT SIDE BEHIND HER EAR. SHE ALSO HAS SEVERE WHIP LASH AND HIT THE RIGHT SIDE OF HER FACE, CAUSING A RED MARK & BRUISING ON THE CHEEK BONE RIGHT UNDER THE EYE. THERE WAS A POLICE REPORT TAKEN WITH MARANA POLICE DEPARTMENT.