How about the VIN, or partial VIN? You might send an email to Chuck Melton at the DeTomaso registry to see if he can search for any info by your father's last name, just in case it's listed and someone's submitted prior owner info.
detomasoregistry@gmail.com
http://www.detomasoregistry.org/
Unfortunately you won't be able to search the registry yourself if you haven't submitted a car to the registry - makes no sense, but that's the way Chuck runs it.
D'oh! The VIN is in the title of this thread - yup, it's official; I'm a moron! Good luck in your search.
Good luck!
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