Here in New Jersey, you can't have an unregistered vehicle in public view. In a neighboring town, the enforcement official tried to lift the tarp from a vehicle to check if it had plates, but the owner stopped him because THAT was illegal. Go figure.
Selective enforcement is another issue. In 1996, when we first came down the road to where our house was being built, there was a 2-door silver Cutlass, in plain view 20 feet off the road, in someone's driveway without plates. The car sat there deteriorating until a couple of years ago before being hauled away. I wonder who the owner knew to avoid being hassled.