When a movie is filmed in a location that is different than the location in the script (e.g. a movie set somewhere in the US that is filmed in Canada), and there are automobiles shown with license plates for the scripted location, I assume these aren't the real license plates for that car. Or they may even have vanity plates that correspond to some element of the movie, like the one that says "YAHOO" on Gordy's Mercedes in the movie Frequency.
So technically, these plates are illegal because they aren't registered to the car. In the case of the YAHOO example, the car is just sitting next to the baseball field so it doesn't matter. But what if the car is driven? I can think of four cases:
- When the car is sitting still, it has the fake plates. On the road, they switch to real plates and take care not to show the plates.
- When on the road, the car is actually being towed (see here for examples).
- They drive with the fake plates, and these is a "don't ask, don't tell" agreement with the authorities since you're making a movie, and they keep the real plates with the vehicle if needed to be shown.
- They drive with the fake plates, and there is some sort legal exemption that covers this.