It is not legal in Australia to mount a baby car seat with only the lower Isofix points & a car seat purchased outside of Australia will not have the required approvals for use in Australia. Baby - child car seats here must have Australian approval tags and be attached with a top tether plus the adult seat belt or Isofix. Isofix alone does not meet our regulations.
But, that said I added top tether car seat mounting points into my 76 series Cruiser by bolting two load rated eye bolts through the floor behind the rear seat. The floor was reinforced with a steel plate under the floor to spread the load. Then I purchased rated car seat extension straps & rated shackles to bring the attachment point up to a little lower than the seat top with head rests removed.
Similar process could put a top tether into the parcel shelf of a vehicle although cars sold in Australia have had top tether mounting points ex factory for 25+ years. But some 4wd like the 76 series cruiser somehow dodged these regulations.
It is not to Australian Design Regulations (ADR) but I believe stronger than factory fitted top tether points and doubt any installer would do it differently.
Most of the baby equipment specialists install car seats and could add the top tether mounts or point you to an accredited provider, eg see
Or buy a $20 drill & do the same yourself! Visit me in Melbourne & I'll lend you the tools.
If my response has missed the mark please
email.me off-line & tell me exactly what you need.
Regards, Graham