Horseshoe Leather shoulder holster.
As I am right-handed, this method puts the pistol in play exactly in the area from which it is most likely for a car-jacker to approach. In fact, if you draw inside the car, you almost have to point out the driver-side window whether you want to or not.
As a secondary reason, you can go to the bathroom without a two-pound chunk of metal on your belt. Unholstering in a toilet stall seems awkward to me.
And as an observation, rather than a reason, reality cop shows such as The First 48 have nothing to do with guns (except as the aftermath of a criminal act), but it is clear that shoulder holsters are a primary choice of duty officers that have the option.
I sometimes have to drive 1,000 miles in a day through the Midwest, and I have never yet felt muscle fatigue related to shoulder carry.
I have also never had to test my theories in real life, and the pistol has only ever been drawn to fire at the range, where shoulder holsters are not allowed.