I find that if you get a car you don't like just go to airport and exchange it. Just give them some reason (wither true or not). I used excuses from the car shakes to touchy brakes to tire pressure light comes on sometimes. The agent inside that swaps the contracts never verify anything.

It best to do it on a Fri or Sat when they have a large selection of cars (in most cities). I usualy drive around the gold stalls and right down stall numbers of a few cars I like and they will give one of them if the car is not assigned. (Used to be alot easier to see which ones where assigned because they would have a tag hanging in them).