So, in order to take the conversation about off-body carry a bit further, especially in the context of vehicles, consider these points.
1) If you have to bail out of a car, anything not attached to you is not coming with you.
2) If you do manage to grab your gun, and the situation changes (deescalate) you have no where to put the gun. True story: About 8 years ago, I was in my POV and got pulled into a car chase. The BG car wrecked out, and I ended up chasing one of the dudes on foot for a few blocks (plain clothes). My gun was in the glove box, so I grabbed it when I bailed, but when I caught up to the dude, he wasn't armed. I had to fight him one handed and keep him from taking my gun, since I didn't have a way to stow the weapon. Now, obviously, you probably aren't foot chasing dudes, but the point stands that bailing out of a car with a pistola and no way to stow it could be a problem.
3) As was evident in the FBI Miami shootout, guns don't always stay put in the car where you want them.
Now, if off-body carry in the car is your only choice, which I can understand, just remember that it should be a really solid mount and have retention. It shouldn't be easily visible by folks walking by either, or in the case of a traffic stop.
Just a few considerations.