WAS DRIVING SOUTH ON I15 WHEN TRAFFIC IN FRONT OF ME CAME TO STOP. I SLOWED MY RANGER TO LESS THEN 10 MPH WAITING FOR TRAFFIC TO MOVE ON. A MAN DRIVING A IZUZU TROOPER WAS NOT AWARE OF THE TRAFFIC STOPPAGE AND SLAMMED INTO THE REAR OF MY RANGER AT APPROX 40 MPH.THE IMPACT SENT ME BACKWARDS SLAMMING MY HEAD THROUGH THE BACK WINDOW OF MY "SUPERCAB" TRUCK. I WAS WEARING MY SEATBELT AND HAD MY SEATBACK IN A NORMAL UPRIGHT POSITION. ONCE I GOT MY WITS ABOUT ME 5 MINUTES LATER I NOTICED THAT MY SEAT BACK WAS FULLY RECLINED AS A RESULT OF THE CRASH. MY BACK WINDOW BEHIND THE DRIVERS SIDE SEAT WAS ALMOST COMPLETELY MISSING AS A RESULT OF MY HEAD HITTING IT. I ONLY RECEIVED A MODERATE HEAD INJURY (3 STAPLES TO HOLD IT TOGETHER) BUT THIS COULD HAVE BEEN MUCH WORSE. SHOULD MY SEAT BACK GIVE OUT LIKE THIS DURING A REAR IMPACT???? I FEEL I WOULD HAVE BEEN FINE IF THE SEAT HAD HELD. PLEASE SEND ME A REPLY WITH AN ENGINEERS ANALYSIS (OPINION). I CAN E-MAIL PICTURES OF THIS ACCIDENT IF IT HELPS. THANKS BRIAN LABREE. *AK