Actually the U.S. Supreme Court held that because operating a vehicle is a privilege, NOT a right, bestowed on you by the state that requires you to comply to a host of rules, regulations and laws, the state is within its right to target ANY vehicle or group of vehicles for safety checks and checks to see if the operators are in complience.
Think back to those thrilling days of yesteryear when you were taking drivers ed...remember how they told you that you were subject to a traffic stop at any time and that by applying for and accepting a license you were agreeing to be subject to the stop?
Now...if you are perfectly legal and have all of the pertinent paperwork and the officers want to search your car or want you to open your trunk...that's where probable cause comes in.
I don't have the time to research this now but I do believe that they must have a reason to pull you over in the first place for a stop, whether or not you, by virtue of having a license, have agreed to comply.
As far as the probable cause, it's as simple as denying them the right to check your car. If they get to the point where they ask you to submit to the search and you deny them, they didn't have probable cause in the first place and if they forcibly search your vehicle, when it comes to court, the police are basically SOL (if they had to ask permission, they never had the right to search in the first place, therefore anything they find, even if confiscated, is inadmissible as evidence).
Let's have a public defender weigh in here, ohh Rob!!
Give us the legality here, who is correct?