I'm waiting to see how this all shakes out - and what side the courts will fall on (although I have my suspicions).
It is my opinion that the 2nd Amendment is the law of the land, the vehicle is private property, and that the employer has no right to dictate to the employee what he may or may not keep in his vehicle.
At work and on the premises is another matter. Although, I am just waiting for the first lawsuit of an employer who denied a CCW the right to self defense, posted the "no guns allowed" signs (an open invitation), and then provided no security for the employees who were then defenseless, sitting ducks. That issue begins the REAL debate!
The employer has no right to coerce you to either own/carry or avoid owning/carryihg a gun. But he does have the right to avoid hiring you, or to fire you. Why? Because of his property rights in his capital. And any suit against an employer based on the idea that an employee was improperly fired due to gun ownership should fail for the same reason that a suit against a private individual that claims damages for refusing to buy product from a particular vendor (for whatever reason) should fail. You have the right to decide whether or not to buy something, and from whom. An employer has exactly the same right--because everyone has the same rights equally.