The dealership will find any way possible to not put fault on itself. The fact that the car has been modified is obviously their reason and it will work. On another note, I can understand your frustration if you told them up front that the car had been modified and they agreed to work on it. It is hard to prove that a dealership actually damaged your vehicle while being serviced. My recomendation is a firm conversation with the General Manager discussing your problem and what can be done to resolve it.