Leave a firearm in your car and it's your liability no matter what. The Courts, airports and gov't building prohibit carry unless you're a LEO. I carry sometimes and am retired from LE. I would NEVER leave a pistol in a car, even in a 2 ton Mosler safe, as you will be held culpible for what ever happens no matter what anyone tells you. Anyone with an ounce of sense, or responsibility would never leave any firearm in any vehicle. Any person shot with your firearm will take you through the court system and it will cost you your house, job, car and any other assets. The crimminal liability is very high in most states and don't count on the NRA coming to your rescue.
Simple solution is to leave them home if you plan to go to court, the post office or any other place they are prohibited. Those "security boxes" can be popped open in minutes if not seconds.
Displaying a firearm in a holster attached to a visible (includes every place other than compartments) part of the vehicle is considered menancing in most states if it visible to any person either out or in the vehicle. Not to mention how police officers feel about seeing a firearm readily acessible to the driver. Life is not a movie and a lot things can happen with very unexpected consequences...especially when firearms are involved.