Of course we all agree that no right is absolute. There are cases of conflicting rights and also situations of health and safety.
I agree with such laws which forbid open sewage, or conditons which cause rat or mosquito infestations, etc.
I think an employer who seeks to ban guns in vehicles on his property is simply wrong, even to the extent of being evil.
What I have in my car unless it is something which is an immediate threat to him such as a bomb or container of anthrax is none of his business: period.
It is the exact same thing as saying you may not bring a car onto his parking lot which contains emergency gear.
His parking lot is his property but it is not private. If it is accessable to the general public, he has no right to make unreasonable demands on those who enter.
The inside of my car is private. I have no idea how the courts view it but inside my car should be considered just like inside my house.
The inside of the vehicle was considered private until recently. No employer could extend jurisdiction to the inside of a vehicle in the lot, except for illegal activities. Even in that case a cause, warrant and cop was required to search it.
A large amount of businesses have decided to challenge the common law regarding vehicle interiors as it stands. The sole reason is to prohibit the exercise of the individual's right to transport and use firearms. There's no mistaking that these employers are attempting to deny a previously existing and well recognized right, previously enjoyed everywhere in the US.