@capsat is very right, If in case the dealer somewhere has the documentation that he sold the car to you, and is a honest man that could be your only chance. Or else theirs no profe at all,
PS: In case thoose plates are used in a terorism act and the orignal owner has proof that he/she sold the car to you, Your a gonner, I dont mean to scare you but this is a very big mistake not getting the car transfered.
One thing with this thread id like to tell.
By law the owner is responsible to get the car transfered on the name of the new buyer. But we always take the easy route out.