I know this is an older thread, but I wanted to share my personal experience...
When our first daughter was a year old we rented a car seat from a car rental place. I don't remember what the brand car seat was, but it was just a cheap convertible seat. First, they tried to install it for us, but it was very apparent that they had no idea what they were doing (they also had to upgrade our car, because they had promised me that the seat would fit RF in their compact car and it didn't). So my DH installed the seat, but we didn't inspect it to make sure it was put together properly, because we were dumb and didn't think about it and weren't familiar with the seat and didn't have a manual or anything. Well, about an hour later, we were driving up a windy mountain road when I looked back and our daughter was leaned all the way forward and practically crawling out of the seat. I screamed and my husband pulled over - it turns out they had assembled the seat improperly, so that when you tugged on the straps, they just came loose! I FREAKED OUT. I was hyperventilating and yelling and calling the car seat company to tell them how they could have killed my child - and they could not have cared less. My husband managed to put the seat together properly (they had completely threaded the straps incorrectly) but we will NEVER rent from a rental company again. As insensitive and ignorant as they were, I just would never trust them - it wouldn't surprise me at all if they used seats that had been crashed, recalled, etc.