I've had a GM Dealer cut a key for me before with no proof of ownership. They kinda had an idea who I was but they had no idea I owned the truck at the time. They said they wanted to see insurance and I told them I was locked out of the truck and needed a key and they cut me one. Maybe most are more strict but it seemed easy enough for me. $50 says I could take a VIN to the same dealer off another D-Max and they'd cut me one without any proof of ownership

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I'm suprized Onstar isn't doing you any good. That's just odd IMO.