Trying to track down my dad's 70 Duster (Bay Area, CA)

I heard a story once about a guy who found his Grandfather's Cadillac. He had an old photo and the Cadillac was in the background with the license plate visible. I think he took the plate number to Secretary of State (this was in Michigan) and they gave him info on it or something. Like I said this was in Michigan. Good luck to you!