G'day Cab,
I just had child restraints fitted to mine. It required a regular child seat bolt (comes with seat) being bolted into the floor pan behind the back seat and then a specialized runner (engineered and rated etc) for up higher. It's bolted at the top of the rear seat and through the rear of the cab. So the child seat belt runs through the runner and down to the bolt.
Did that make sense?
You might be able to get this to fit yourself, but I got mine fitted and supplied with an engineering certificate for $300 from Nissan in Nowra.