I posted a response to the car carry question while ago. Here is a slightly updated response:
As someone who experienced two car accidents, I can say that everything that is not properly secured will become airborne and has the potential to injure or kill you or someone else. A screwed down SERPA holster with its active retention is fine. A holster with passive retention is probably the worst choice. It also should be out of sight from someone standing next to the driver door, like someone walking by in the parking lot or a police officer during a traffic stop. State laws should be obviously followed as well.
Never leave a firearm (loaded or not) unlocked in your locked (or unlocked) vehicle. It is bad enough that I need to even have a GunVault Nanovault in my car, if for some reason I need to enter a gun-free zone and leave my firearm in the vehicle. Leaving it visibly behind is just asking for it to get stolen. Even leaving it behind unlocked inside the locked vehicle (e.g. in the center console or glove box), makes it still vulnerable to a smash and grab job.
I personally carry on the body (4 o'clock) in the car and practice the draw. The most important lesson here is not to sit on the shirt that covers your concealed firearm. Instead, sit down in the car, close and lock the door, and lift the back of the shirt you are sitting on above the firearm and between the holster and your skin, while keeping it concealed from the outside view. The draw requires me to only slightly lean forward to the left to access the firearm and remove it from the holster. It is slightly slower than drawing while standing, but keep in mind that you do have another option: Drive over the threat if a legal self defense situation with a deadly weapon exists.
Here is why I don't advise to go the mounted holster option: What if you do get involved in a car accident? Will you leave your firearm in the car? Will you holster the firearm first, before exiting the car? What if holstering your gun, i.e., having the gun in your hand, right after an accident is perceived by someone else as a thread? Too many variables to be worried about in a stressful situation. The only thing I have to worry about is accidentally exiting the car after a crash in open carry vs. concealed carry mode. Since my state does not differentiate between both, it is perfectly legal for me. Note that all of this also applies to other situations, such as when you need to exit the car to help someone.
PS: I had one "positive" experience during the last car accident. A women ignored a stop sign and just drove across the intersection, putting her car right in front of mine. With a total of 4 people in my car, there was not enough room for me to come to a stop. I hit the other car in the back of the left side, turning it 180 degrees. No-one was injured. Both cars were totaled. Now, here is the "positive" experience. I had an adrenaline dump, experienced tunnel vision targeting the threat, and perceived time as being slowed down. All effects someone would experience in a serious self defense situation. Probably the best "training" of my life, costing a total of $25,000 in property damage inflicted by the other driver.