Wow... what a corker.
What southo says is true, for someone to pay then there needs to be someone who is negligent. Now the people ( I'll keep it clean even though thereis another word in my head that starts with p) that cause the damage cannot be located to foot the bill.
So, if I understand it correctly, and correct me if I'm wrong, the blame falls on the next MOST negligent party, which, seeing as you had left your car in the care of the pannel shop would be them. If the car was parked in your garage at home then it would be you.
Speak with your insurance company... tell them everything. They ARE there to fight for you, you don't only pay a preimium just incase you muck up... but also to have some punch against others that might stuff up.
Good luck man... I feel for you.