Sorry, the only dark blue GT6 Ive seen is this one but its a 67:
https://cgi.ebay.com/ebaymotors/ws/...&rvr_id=&satitle=220518956105&fvi=1&viewitem=
And due to its age, its possible that it your moms may been repainted once more. But I'll keep my eye out for one matching that description, and wish you luck.
My mom owned a 1971 yellow VW, bought it new. A year ago, I found the actual receipt from the dealer for it, including VIN. I went to a forum, much like this, and someone found it for me. The address and google satellite images confirming, but it was an hour away in a not nice area, so I never went to confirm. Plus, its not like I was going to buy it. At least not that point in my life. Well, that forum had some data loss, that thread including the VIN and location was forever gone, and I no longer had the receipt with VIN.
As far as "civilians" go, someone did what is called a "vehicle inquiry" on my very first car. We had sold it 8 years ago, but was left abandoned by someone who was not the buyer, and this other person wanted to buy it. It was never retitled, so after filing a loss title, we were legally the new owners again. So it is possible, but you need a VIN. Does your mom have any tax boxes from way back when? Thats where I found my moms VW VIN.
Without knowing the VIN, there really is no way. Especially since the car may not have a current registration. My GT6 hasnt been registered since 1993. Since then, it has traveled many places, and had many owners.