Sounds like your car is out of town. I would go out there unannounced and see for your self were the car is at.

Whenever it's MY money is in a project ... I control it, not the hired help. Progress payments yes, but quality is more important than completion date. This is art to some extent. Your car differs from others if the artist goes out of his way to do a great job. Artists cannot keep to a project timeline unfortunately.

Make sure the quality is there and you don't have him over paid his advances. If he has little or no advance payments then that could be why you are the last car to come out of the shop.

Offer a $$ bonus to complete by XXX date...if the quality is there and not much left to do that is critical.