Why are we holstering and unholstering to begin with?
I'm with chezuki on this one...if it ain't on your person, in an emergency, most likely, it'll still be in the vehicle if you have to egress in a hurry!
While in a vehicle, summer carry, it's in an OWB RH 3-slot holster in the 4:00 position with my shirt pulled up over and tucked-in behind it while driving (under the seat belt) for quick access.
In the winter, I prefer 3-slot holsters because I carry OWB 10:00 position because I can unzip my vest (preferred winter dress...which I can, typically, leave it on indoors...or coat over a vest, which I can also unzip part-way) and have access to it while I'm riding/driving in a vehicle.
Keep in mind, if the $h!t really hits the fan, and you're in a parked vehicle, you'll want to be able to exit the vehicle very quickly...and be able to move around and use it as a shield (behind wheels or engine area) if possible. If you remain in your vehicle, most likely you're in a 'fixed' position and your assailant is mobile...meaning he can move around (change your angle of defense) while he can change his angle of attack...not a good situation to have to defend.