Avis must get a VAG for electrical inspection

We hired a car at avis, we waited for the car upon its arrival we inspected the car.. All was good visually.. Drove off boom coolant light come on..ok refilled it continued on our trip coming back same light.. We then contacted avis who then asked to exchange the car..which they did Fine.. After our rental we returned the car.. After sometime without getting back deposit we went to their offices.. n was told its been used to pay for excess because it looks like coolant is leaking so they are going to change the radiator, I asked how is it my fault. Was told their psychic technician THINK it was damaged by a little stone thrown by someone on the road.. N I must be charged..what a service