Mercedes damaged my car during a service and deny any liability

Took my Mercedes A200 for a service on 2018-05-11 and left it over the weekend for service until 2018-0-14. When I received the vehicle I drove to my wrok for the week but noticed when I got there, it was damaged. Damage that could only have been caused to the back of the car by someone bumping into the light with a sharp object! One of the lights was cracked. I contacted them, but they claim that as my vehicle came in dirty, no assessment could be made beforehand, and that it could have been damaged when I booked it in. Never take your car to these guys as they are not to be trusted!!!!!!!