I think we're down the wrong bunny bath. Wasn't the .357 mag intended to defeat the body armor of the day? What it might do with a vehicle would be incidental.
The vehicles were stout. I've read an account that one of the officers involved in the Bonnie and Clyde ambush examined a Barrow vehicle from an earlier shootout. He learned the .45 acp slugs from Thompsons did not penetrate the car bodies and supposedly went and procured a BAR some time thereafter.