the other option is you can build a taxi bar across the rear parcel shelf using the rear seatbelt mounts to pin the bar, you will also need a bar to stop it from rolling generally locked down at the child restraint in the centre of the shelf (if you don't have a CR you can use a 3"x3" plate with a nut welded under the shelf to attach the antiroll bar.
Then the harness simply wraps to the rear taxi bar crosses over in an X to your seat holes ....
I had mine engineer approved with my mod plate along with everything else. he said it is probably better then a behind the seat job.
you also need to be aware of the angle the harness rolls over the seat, your little book of rules (Main Roads acceptable mods) will tell you.
I think it is 5 deg up and down to 45 deg but better check on that.