So,....let me get this straight.
The douche bag went into work, during non-business hours and took the OP's car for a little "joy ride" and ended up totaling it. If that's the case, the douche bag is 100% responsible for this. The dealership is like a third party issue. Yes, the car was housed there overnight, but the dealership did not allow the douche to take or steal the car for the day, non-business hours. The douche decided that on his own.
It's just how I see it. Kinda like suing Burger King because they make fatty foods, I eat their fatty foods and I weigh over 350 pounds now.(Not really, but you get the idea. lol
)
I hope all goes well for the OP.