Nonsense.
In no other instance does a American’s civil rights end at anothers property line. Furthermore, an American’s vehicle being considered an extension of his “castle” is a well established legal doctrine.
What really is at issue here is the civil liability of an employer for torts committed on his property by a gun wielding employee.
Simply exempt the employer from civil liability for the employee’s firearm and the whole problem goes away.
I don't even know what that means. And it is not a "well established doctrine". Certainly a person has a privacy interest in a vehicle to be free from unreasonable search and seizure from agents of the state. The right is much less than with a home as demonstrated by the legalilty of a "Terry" stop and search. But any employer would have the right to require consent to search as a condition of using a parking lot. The employee can refuse and work, or park, somewhere else if that is his desire.