And 'round we go...
Actually, this is a great situational example for debate. Some of the best threads on this site are discussing conflicting rights, IMO. Particularly when the rights involved are historically conservative.
As for conflicting Rights. I really don't see it that way. The Employer has his property. I, as employee, have mine. As long I as own that vehicle, what is inside it is my property. No matter where that vehicle sits, the contents are mine. No one elses.
If an employer says "No firearms on MY property", then fine. My guns will stay in my car. While on my employers property they will never pass the boundry of my property on to theirs unless such circumstances would save my life or the life of another. In which case, I would feel comfortable suffering any punishment for violating my employers property Rights.
Staying alive is my first priority though.