Since you were close to the prison, the vehicles probably were probaly headed there. You might send your letter to the warden of the prison. Chances are you will get a better response from the prison than the governor. The prison would probably be able to determine what vehicle it was and track down the driver and passenger since there are probably records of when vehicles enter and leave the prison.

The governor's office might send your complaint to the prison (if they make the connection) but I wouldn't rely on it.