And... the "Texas Problem*" rears it's ugly head.
Look... just because it's one way where you live (in this case, Texas), doesn't mean it's that way everywhere. Different states (and sometimes even municipalities) have differing laws. And more than one police officer has gotten in a bind for not realizing that (of course, so have many citizens).
Trust that someone from outside your area may be under differing laws, and may even know their local laws better
* I call it "the Texas Problem" from my time on the gun boards. Go on almost any gun board, and relate an occurance where your legal CCW was exposed. Within the first 2 pages of replies, someone from Texas will post about how you could have gotten busted for "brandishing". Never mind many places now have open carry: visible handgun in Texas = charge of brandishing, therefor it must be that way everywhere
No, not a Texas thing, this would be an everywhere thing.
The owner or leasee has rights to that property, which can dictate what you can and cannot bring onto property, that would be everywhere, it is part of your rights as the owner, hence why in some states laws are enacted which defy that right, see post in this thread about OK having a law allowing firearms in parking lots of employer, this would be an example of a law enacted which defy the rights of the property owner.
Lets take this into an easier to digest example:
You own your own home. You have a friend who delivers chemicals and decides to stop by one day and say hi, and does it in his work truck. He parks in your driveway. You tell him "hey I don't want those chemicals on my property, you need to leave". He refuses. His refusal to do so now constitutes trespassing, a law which you also have in WI, not just Texas. You as a property owner have a right to dictate what is on YOUR property, just as your employer has a right to dictate what is on HIS property.
So back to the topic of discussion:
Your employer has the right to do the same thing, it is his property. Unless a provision has been written into the law specifically allowing you to bring something onto his property, they can dictate what you can and cannot have, even in your car. Normally with most employers, they have already told you what you can and cannot bring onto their property, refusal to do so will probably get you fired, refusal to leave after that firing will more than likely get you arrested, see above for trespassing.
The precedence you are referring to that deals with your vehicle as an extension of you and covered under the 4th amendment only deals when you're dealing with government officials (Police, FBI, DEA, etc, etc), not your employer.
I'm not trying to be rude or arrogant, but law is what I deal with on a daily basis. I'm all for a good debate on the subject, but as I said previously in this thread its important to under stand that your rights protect you from the government, not your employer, he has property rights as well.