There are brackets that should be bolted to the floor of the VX right where it turns to slope down to the foot well for the rear seats. They should stick out above the carpet so maybe the previous owner removed them along with the rear seat. Once the brackets are in the rear seats are held into them by a large pin with some plastic "bushings" around the pin. I replaced my stock pin with a removable pin with a safety clip on it so it doesn't come out by accident. There is also a metal "loop" bolted to the floor towards the back of the VX that the rear seat locks to once it is folded down. The rear seat belts are connected to the bracket that the loop is welded to.
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