I read a little thing on strategies and tactics by Box o' Truth a while back, and it influenced my thinking on car carry.
If my car is being forcibly entered, or attempts are being made by a perp, or I am drawn on or shot at, here are my options in order:
1) Pedal to the metal. E&E. Lay down fire to keep the perp's head down if necessary.
2) Car as weapon. More effective and easier than a gun, every time.
3) If car is blocked in and I am about to be shot, get out of car and escape or return fire.
4) If I can't get out of car, can't leave in car, and can't use car as weapon, shoot from car and attempt to stop the threat.
Because option 3 & 4 are the worst case scenarios, and option 4 is the only one that requires a fast draw from inside the car, I choose to carry on my person while in the car - "wear my gun" as sm and others call it - simply because I will probably not be using my gun from within my car if I can escape using the car, or use the car as a weapon instead. If I have to use the gun I will probably be on foot.