Hi,
We just returned from a trip to Tucson and had a rather interesting experience with car seats. We rented a minivan from Thrifty, but brought our own car seats with us. Well, when we went to install my 4 year olds car seat, we found that the Town & Country minivan did not have the ratchet type seat belts (where you pull the seat belt all the way out and it ratchets back to as tight as you can get it) and we could not get the seat belt to secure enough to feel safe driving around with him in it. After the Manager came out and reassured us that the seat belt would lock in the event the brakes were applied quickly (????), we checked a few of their other minivans. We ended up using one with the built in car seats which is something I would look for in a rental the next time. It takes up much less room in the car and had the 5 point harness that our car seat had.
I'm not sure what we would have done if we did not find one with the built in seats b/c I know we would not have left the lot with his seat moving all about. His was the type where the belt hooks through the back of the car seat.
I know I didn't answer your question, but you may want to call ahead to see what type of seat belts you car is going to have. We have never run into this problem before b/c we've always had cars with the ratchet seat belts.
When we have used the car service car seats for our trips to Disney they have always been very nice and in good shape, but then again, they are only used for quick trips to and from the airport.
Good Luck!!