I would park that car at the end of a very dark alley. Come back in a couple hours to get it. When they ask what happened, just explain that you guess they should have had your car done on time..... Well, assuming it still has wheels. If not, then just explain that the Tart is ready for pickup and give them the location.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car