I recommend that you always carry the weapon on your person. That way you always know where it is. When you get out of the car, it is with you. You never know where and when a threat will surface. Find a weapon that is easy and comfortable to carry like a smaller Glock or S&W Shield. I recommend 9mm or .40 cal. Those rounds have plenty of stopping power and you can usually carry more rounds more comfortably. Get an extra magazine and carry that where it is also accessible. Don't be fooled by a larger caliber because it is all about shot placement. The threat will keep coming if your shots are not on target. The more rounds you can send to the target, the better chance of neutralizing the threat.

As an LEO, I have seen many weapons stolen from cars because well meaning gun owners who "forgot" their gun in the car. That's just another illegal gun on the streets for us.

Once you pick a comfortable gun and holster, PRACTICE, PRACTICE, PRACTICE. Muscle memory is everything when faced with a stressful encounter. We practice all the time. Practice drawing while seated in the car. (unloaded of course). If a threat is coming at you, shoot right though the windshield. The bullet will only make a hole and will loose only a small amount of velocity. I have done it.

ALWAYS let an Officer know when you are carrying a weapon and wait for commands. Stay Alive!

Not trying to insult anyone's skill level, just trying to offer good advice from our side. Most of us support responsible gun ownership.
Stay safe everyone.