When I was at Blackwater in September, we did a media demo (no not the news media) where we shot up a car from both inside and out with different calibers. The long and the short of it was that it really depends on where you hit the car, and in most cases, you can't predict the "thin" spots. Bottom line was that the randomness of the internal stiffeners, motors, etc made it impossible to say with any certainty where a thin spot was. You could shoot it with a .308 without complete penetration, and only 1 or two inches to the side was thin enough for a 9mm to go through both sides of the car...
We shot a SAPI insert
The cool part was I was the only dude with a .45, so I got to shoot at some targets through the front windshield to demonstrate how the glass deflects the bullets...
Then we blew up the car with Tannerite...(1/2 pound)
bitchin.