Rear seat belts torn

Hi. We purchased a second hand 2009 Prius about 8 months ago. A month or two after we purchased it we noticed both the left and right seat belts in the rear seat were torn. The tear was so straight we assumed they had actually been cut and put in an insurance claim. Three months after having the belts replaced we noticed they had ripped again in exactly the same place - obviously a fault rather than a malicious act as we had originally thought. We took this to a Toyota service centre to get it repaired under warranty and to get the fault fixed. They couldn't find anything wrong but, after much discussion, they reluctantly replaced the belts as a good will gesture. Just two months later it has happened again! We can't believe it. I've found three other similar complaints on other forums but none appear to have been resolved. The tear is occurring 1-2 cm BEFORE the belt goes into the retraction mechanism and therefore the service dealer argues it can't be a fault in that mechanism. We have never put the seats down, we rarely carry passengers in the back seat and we never carry anything sharp in the boot that could cause this. Since we last had the belts replaced we've been checking them regularly, in fact we had only checked them a few days before we noticed the tears, so it doesn't seem to be something that is occurring after repeated friction. Has anyone encountered this issue before? We can't afford the cost of constantly replacing seat belts and wouldn't want to sell a car with a serious safety fault.  Toyota is simply refusing to look into it any further so we are at a loss as to what to do. 

 

Thanks for your reply. We haven't had anyone in the back seats between the time we last checked the belts and then noticed they were ripped. Have checked the belt buckle thing but there don't seem to be any edges that are rough enough.
I'd like to send a photo - how will I know when I'm allowed to do that?