It is a Fourth Amendment issue.
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.
What right does a company have to search private vehicles? Absent probably cause, not even the police have that right.
Dear LouD,
"What right does a company have to search private vehicles?"
The Fourth Amendment protects you from government action, not private action. A private employer actually has more right to search than the police.
That's not to say that the private employer has an unlimited right to search your person, your possessions, or your car.
However, a private employer could make, as a condition of employment, a rule that required you to submit your car to search at any time, when on company grounds. A governmental employer would have less freedom in making such a rule, although here in Washington, it is now policy that if you bring your vehicle onto government property, you cede to the government permission to thoroughly search your vehicle.
In general, you have the right not to park on company grounds, or not to work for that employer. In general, you don't have the right to prevent your employer from making such a rule a condition of employment.
sitetest