Door Latch problems of the 1999 Chevrolet Lumina

I have a 1999 chevy Lumina, that in 2007 both the rear door latches broke a few months apart I contacted the dealer both timeas and had the latch replaced at a cost of 2 approx. 200. 00 each I had asked if there was a defect in the part once the same thing happened to the other rear door they replied no. I thought this was strange since I only use the veh to go to work and rarely use the back doors. The first time this happened my aunt was in the rear seat when I went slowly around a bend and the door flew opened and would no latch to close. I expressed my concern about the safety of the latch letting loose without warning, and the a few months later the same thing happened while driving luckily no one was in the back seat. After having both door hinges replaced with new parts at a cost of over 400. 00 I thought that would be the end of the problem. Today while coming home from work the rear passenger door flew open, I have concerns for the safety if someone would be in the veh, gm does not seem to share my concerns and seem not interested in trying to resolve a safety issue.