if their prints are in the car, that can be easily explained by working at the dealership.
if they were smart/sober enough to take the plates, the should be smart enought to get to the dealership to do what boots said.
just cause the plates were taken off the car doesn't necessarily mean it was the dealer. the thief's could have taken them for future thefts to prevent getting pulled over?
but it sounds like someone at the dealership was involved.