Shoulder holsters are too easy to get tangled taking them on and off. Sometimes one side is looped through the arm-hole, the whole thing is backwards, both sides are looped.... You will find it very frustrating to don from a sitting position inside the car.
I suggest a paddle style holster. They don't conceal well, but then concealed isn't on your list. You can even use a paddle holster without a belt as long as you aren't going to be running or going inverted (assuming the gun isn't overly heavy). Some of the plastic paddle holsters have barbs to hold them in your pants so they don't follow your gun out, but those can be filed off. A tightly fitting holster or one molded tightly to the gun may give you some trouble when you try to draw, so you may need to tinker with it (thus rendering it to your specific purpose only).
In this case, I would say go cheap. Get a 'universal' paddle holster and make adjustments to the holster and paddle until you can easily put it on and take it off from behind the wheel of the car.