I would think rear seat belts were available in 1965...I know they were available as options at least as back as 1961. I put belts in my '61 Bonneville (since it didn't come with them), and after doing some research I found out the factory put little dimples in the floor pan to show where to drill the holes for the retaining bolts.
They were pretty subtle, so I had to look for them for a while - check your car, they probably did the same thing.
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