I don't recall language in the Bill of Rights that says certain amendments and the rights enumerated by such are superior to others. Perhaps you can point that out to me.
The particularly nasty thing about parking lot prohibitions is that they effectively prohibit subject employees from exercising their RKBA rights far beyond the parking lots in question. Two very substantial reasons for concealed carry in a vehicle are defense against car jackings and being prepared to deal with a burglar or home invader if, God forbid, you run into one on returning home from work. A few years ago, near where I live, a couple was murdered by a burglar they ran into on returning home. This fundamental personal safety reason is why an employee can be justified in violating such an asinine rule, just as he or she could reasonably violate a rule prohibiting use of the bathroom when he or she had stomach flu.
Thus, there is a powerful conflict between the employer's property right to prohibit guns on its parking lots and the employees' rights to their own personal safety. Equally important, the argument that allowing legally carried guns in employees' vehicles increases the danger of a mass workplace shooting, is bogus. In fact, as shown by the work of John Lott, the best way to prevent such shootings is to allow employees to exercise their RKBA rights to keep guns in their vehicles.
In other words, it is objectively, scientifically provable that an employer reduces its risk of workplace shootings by at least allowing employees to keep guns in their vehicles. It's not only a fundamental freedom issue, it's also a vitally serious safety issue.
Governments customarily regulate employers' workplace safety practices by, for example, fire codes and OSHA regulations. Requiring employers to respect employees' RKBA rights is far less intrusive than most present, generally accepted regulation and decisively promotes public safety and workplace safety. Those on this thread arguing for the sanctity of employer gun prohibitions are probably great supporters of far more intrusive governmental workplace regulation than merely protecting employees' rights to carry in their vehicles. Hence, a statutory requirement that employers respect employees' rights to carry in their vehicles is entirely reasonable.