[Angrybird 12]

To me this is all on the dealer. They have a responsibility while the car is under their care not to incur further damage from their negligence. You may have to get a lawyer and sue the dealership for negligence, loss of use of the vehicle, and maybe for your aggravation. GM probably will not replace the car themselves but for customer relations they might. So I would continue to persue both avenues and see which one leads somewhere.
Good luck.