I used to be concerned about my ability to access my weapon while in a vehicle. I always made sure my covering garment went over the seatbelt and I never had an issue. I spent a few hours sitting in my garage and practicing my draw stroke. Of greater concern was striking the steering wheel when addressing a threat on the driver's side.
Over time, I had the opportunity to get some training and to put that training into practice. I had an overseas deployment where there was considerable attention focused on how to deal with a threat when you are in a vehicle. Based on my experience, I am not sure being able to access your firearm in the vehicle is as big a deal as I once thought or everyone seems to believe it to be.
As has already been mentioned, driving out of a problem is generally your best defense when in a vehicle. Leaving yourself enough room to maneuver, adjusting your mirrors properly, locking your doors and remaining alert will provide a greater degree of safety than a special "car-carry" holster. Even the smallest cars have a surprising amount of power that can be used to remove you from a bad situation. I had the opportunity to observe an older chevette push a Ford Expedition about 40' into an intersection from a dead stop. (Both occupants were uninjured.)
If I am in my car and I need to access my weapon, it is probably because my vehicle has been disabled. Exiting said vehicle will be a priority and a pre-requisite of exiting the vehicle would be to remove the seatbelt, eliminating the issue.
When entering your vehicle:
1. Lock doors (you have now placed a barrier between you and a potential threat but you retain the ability to access your weapon.)
2. Start vehicle and place in gear (you can now drive away from a problem; this is now your primary method to deal with a threat requiring deadly force)
3. Seatbelt on (restricitng access to your weapon) I frequently put my seatbelt on as I pause to exit the parking lot, not in the parking space.
When exiting your vehicle:
1. Seatbelt off (improving access to my weapon)
2. Vehicle in park, ignition off (weapon is now my primary method to deal with a threat requiring deadly force.)
3. Unlock doors and exit vehicle.