It's BS. I would ask to speak with the Owner of the dealership. I would explain the situation and explain that you really want to settle this without legal involvement. As far as "driving it beyond it's limits", what exactly is that supposed to mean? So, if the mirror falls off it's because you are driving it beyond it's limits? They designed the car to be able to function when driven, and after all, it's a mustang. If it fails, then the warranty needs to cover it. Now I could see if you put after market parts on to make more HP like a blower or nitrous or something, but sounds like you