amchugh: "Second amendment rights trump private property rights? I don't think so, but it'd be nice if it were so (for 1st amendment too!)."
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I'm getting tired of explaining that this is a Fourth Amendment issue, but...
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You have this all wrong. It is a private property issue: your car is your private property and (for Fourth Amendment purposes) is considered to be an extension of your home. And like your home, the police must have a warrant to examine (search) what is inside of it.
No private party has any right to define what may be (legally) secured inside your home or your car. The employer's property rights end at the point at which your tires contact his pavement.
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U.S. Constitution: Fourth Amendment
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Employer's rights only extend to telling you whether or not your tires are allowed to contact (park on) his pavement -- or whether or not he will employ you.
And if he conditions that permission on what is in your car? Let's take guns out of the equation. Say an employer makes a rule that you cannot leave valuables in your car in his lot, because he believes that would attract thieves. You agree, but decide to leave some jewelry in the car, because your work precludes its wearing during work. Your boss discovers that, and terminates your employment for violating his rules that you agreed to.
Valid or invalid? Is your fundamental right to own property superior to his fundamental right to decide what is allowed on his property?
Police make warrantless searches of cars all the time. I agree that it should be an extension of your home, but that is not how the fourth amendment is interpreted in today's legal system. In CA at least, you waive your right to not have your car searched when you sign the little form for your driver's license. Although you should not be able to waive your rights, there it is.