Where does the employers rights end? Can he demand that you submit to cavity searches, strip searches, Sexual intercourse?????
I would think that your parked car would be private property and that he could not require you give you give up your rights. If they brought in a drug sniffing dog and it alerted authorities to a particular car. Would the authority to search without consent be due to law enforcement activities or the employer's property rights?
I find it hard to believe that liberal judges are willing to support the rights of terrorists to kill us but you don't support the right to self defense.
He can demand anything within the law, and you can refuse to accept his workplace rules and work elsewhere.
If the employer sets unreasonable requirements, then he will not be able to secure sufficient employees to run his business, and he will be forced to either change his workplace rules, or close down.
Look at it this way.
The employer gets to set rules regarding access to his property by others, as you do with your private property; property rights are the basis of all our rights.
You have, as the employer has, a right to defend yourself, not a Constitutional right as other have argued here, but a right to self defense nevertheless, and your right to self defense is inviolate as is the employer's.
You feel that the best way to defend yourself is by carrying a loaded weapon in your car, or perhaps even on your person, and that's YOUR right.
The employer feels that the best way to defend himself is by not allowing guns anywhere on his property, and that HIS right.
Thus far it seems that your individual right to self-determination of self defense cancel one another out, so what then sets the standard?
The right to self determination in one's own property.
Theoretically, all employees are informed of every existing work place rule prior to their accepting a job; accepting a job implies that you accept the rules of the workplace right along with the wages and benefits.
Legally, the employer has further rights to change old workplace rules, or to implement new ones, as long as he makes a reasonable attempt to inform all employees of the changes.
The fact that the gun is IN your car does not make it OK to violate the property owner's rule, because the car is ON his property which violates his right to determine what he feels is his best option for self-defense.
The fact that you believe that he is wrong in that aspect doesn't really matter, because his right to self-determination as it relates to his own property trumps your opinion of his decision.