Ok, a rather strange question here. I couldn't find anything on any websites about it. I have a Ford EF Falcon (class MA vehicle) which would normally be 35% on everything apart from the windshield which would be 70%. Now, say I fully replace the rear seats of the car with something like this http://img.photobucket.com/albums/v518/meade916/honda%20rebuild/DSC07351.jpg .
Clearly you can't see out the back windows so tint darkness shouldn't matter. This has been done before in NZ so it must be fine. But would I need to register the car as a different class?