In addition to the above points made, I would say scan, scan, and scan.
It is very easy to become so focused on the guys at your side windows that you miss the guy behind the car or even the guy with the black j-frame right in front of your face on the otherside of the windshield. He may mutter something about people w/ NRA stickers being the easiest to kill

I would avoid any verbal exchange, other than a single simple verbal command, with the people who appear around your vehicle. Talking ties up your brain and slows you down. Remember the OODA loop.
Be ready to move out of the line of fire as soon as strange people approach your vehicle.
Know how to get out of your seat belt and access your holstered weapon simultaneously.
Get below the dash and be ready to exit the passenger side door. Make sure you put the car in park before you start moving.
Practice drawing your pistol from its normal carry holster, and moving it around the steering wheel at the 3-12-9 o'clock postions if your right handed.
Some other points to ponder. The windshield will spider web when shot, and the passenger windows will just drop away. The seat belt will not retract if you are in a collision. So move the seat belt all the way to the door when you release it.
Place your off hand under the chest strap to access the seat belt latch and then move the belt to the door. At the same time, use your dominant hand to clear the concealment garment and draw the weapon.
Also, firing a gun inside a car might be a tiny bit loud. Sticking the muzzle outside the window may help with the noise.
Locking your doors as soon as you get in your car is also a good idea.