If you're using FMJ's any handgun round greater than 9mm Luger will penetrate a car door. Why are you shooting at people in cars? If they're trying to get away it's hardly self defense. If they're trying to run you over shoot through the glass or get out of the way. If they're shooting at you for inside a car shoot back. I doubt during the stress of a shooting incident you'll even notice if the rounds are hitting the door and then the person shooting at you.
http://www.theboxotruth.com/docs/buickot3.htm
Dave
Any handgun round greater than 9mm? A G34 with 124 NATO penetrates as well as MOST carry handguns (9mm - .45) when shot with standard ball ammo.