You have a concealed permit that allows you to carry your loaded firearm in your vehicle? I have never seen one of those. All I have seen required the firearm to be ON your person. You can travel in a vehicle, but the gun has to be on your person while doing so. A loaded gun stored in your vehicle(within reach I assume) and not on your person was contrary to the stipulations of the permit regs I have read, and a concealed weapons violation even if you have a permit...
I have not really followed how some of the court cases have come out, but I know the issue of conflicting private property rights, both for the employers parking lot and the employees vehicle have been or are being brought to court. The initial interpretation that I saw is that basically it is their property and you neither have to park on it nor work on it.
It has been a long time since I had a concealed permit, but in the last state I had one, I recall the stipulations for concealed carry prohibit carry in national parks, large crowded areas/gatherings, places where alcohol is served and any private property that prohibits such practice.
The issue that has been left hanging and why this is being challenged in court, is that if they deny your right to carry, or leave your gun in your vehicle(for protection to-from work), how are they guaranteeing your safety, particularly while traveling to-from work? Also if a person who didn't follow the rules was to be so inclined, how difficult would it be to walk into your place of work and start shooting? Virginia tech was a no gun zone, that guaranteed a nut case an undefended shooting gallery. I havn't heard of any lawsuits from students who were denied the right of self protection yet, but I would not be surprised by one. What most don't realize, or the people who dream up these restrictions will willingly admit, is that permitted carriers are not a threat(look at the documented crime rate amongst permitted carriers). They are typically responsible citizens who follow the rules, hence the permit!
There may be something more about these type cases on NRA's website.