Here is the issue. If the police had articulable suspicion to pull your car over and tell you the plate was not visible, then the police had the right to ask for insurance. If they asked and you refused and they had a suspicion something was amiss, they can ask you to step out of the vehicle and search your car. Here is the problem: you need to prove your plate was clean. If you can prove your plate was clean or visible, then your attorney will then seek to supress everything after as a result of it being the fruit of the poisonous tree. The poisonous tree being the illegal stop to begin with.