Pretend we're doing roadworks on the night, get people used to the sound of engines in the middle of the night. Then get the cossies. I'm sure we can get some High visibility jackets and rent heavy machinery for the explicit purpose of leaving it on in the middle of the night.Three men per truck, one truck per car - all need to be rollbacks because we don't know for certain which ones will roll on their own wheels (plus breaking in to undo the handbrakes takes time)
One man to keep his ass planted in the driver's seat, one to run the hydraulics and winch, one to chain it up.
That's not a heist - you're looking at AT THE VERY LEAST an hour in and out because of the tight spaces and the cars being parked in clusters. Which probably means you'll have to murder anyone who shows up to question you.