There's really no trick to clown cars, according to Car and Driver. No trap doors, no hidden panels, and the cars are made from real cars. The vehicle's guts are totally removed and clowns are crammed in every which way. In fact, they even know exactly how many clowns different car makes and models will hold (the original VW bug holds 17 while the original Mini Cooper can fit 21).
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Nobody is every likely to prove this conclusively since the artist never told.