So the answer is, if someone parks a vehicle on your property, then yes, you can break in to it. However, just like the government when conducting search warrants, you are responsible for the damages.
I understand completely about the 4th amendment being a restraint on government. I also understand a thing or two about vandalism, burglary, etc. If I park in your driveway with your permission, you have no right to search my vehicle arbitrarily. If I park in your driveway without your permission, you have the right to have my vehicle removed from your property. If you break into my vehicle to search it, you are liable for any damage you cause, which belies your "right".
The USSC has repeated upheld such cases.
Relevant cite, please.
Here's how it works in my jurisdiction if a car is abandoned and a property owner wishes it to be removed:
1. Property owner calls requesting us to move the vehicle.
2. We call a towing company to come and tow the vehicle to an impound lot.
3. We open the vehicle with the aid of a locksmith if necessary. His services become part of the charge to get the vehicle out of impound.
4. We conduct an inventory of the vehicle's contents.
If you as a property owner smash the window and search the vehicle, you can be charged by the vehicle owner for burglary and are liable for anything that he or she claims is now missing. You have assumed what is called in legal terminology a "bailment", in other words, you are now responsible for that vehicle and its contents.
You have asserted your "rights" as a property owner, but they come with responsibilities. If you find evidence of a crime, say, a pound of drugs or stolen property, you have just presented the investigating authorities with a very sticky problem. It can be claimed that you planted the contraband and you can be charged. At the very least you will make an investigating officer very angry with your ham-handed spoilage of what could have been a legitimate case.
Your best bet is to have the vehicle removed from your property without disturbing the contents. Just because something is on your property doesn't mean it belongs to you. If you were to decide that you needed to break the windows on a vehicle that belonged to someone else while it was on your property, they would be justified in using force in stopping you. Your "rights" as a property owner in this case extend essentially to choosing what may and may not be present on your property.
If you sign an agreement on employment authorizing the company to check your vehicle for theft of company property, you may renege on that agreement at any time and refuse. The company may then terminate you for cause, but they may not use force in accomplishing their "search". That's where you're having a problem. Ultimately it comes down to who is authorized to use force in carrying out a search. If consent is refused, absent a signed search warrant executed by an LEO, private citizens or companies have no authority to use force to detain or search another person without probable cause.
As a private person you don't need probably cause or even reasonable suspicion to conduct a search.
As a private citizen you can and will be charged with assault and battery or unlawful detention if you use force in conducting a search under the circumstances you describe. It's you that doesn't understand the workings of the law. Absent probable cause, you have no inherent authority to search persons or property under their control simply because they happen to be on your property.