My Patrol didn't come with a heater as well. It was a "luxury" item.

In my case I installed the complete heater assembly from a small Datsun pickup. I brought the heater along with the Patrol into the States. For this reason, I've wondered a number of times about some aftermarket heaters, since they're probably more efficient.
I had this link from Jegs saved for a case like this:
http://www.jegs.com/p/JEGS-Performance- ... 7/10002/-1
What I will recommend is making a mock up with cardboard with the same dimensions of one of the heaters and see how much space you're left with. The Patrol heater has the blower in the engine compartment, and the radiator inside the cabin, so it compromises less space.
Take a look at this picture from mine. This is when I was doing the setup to find the best orientation. It's tight but allows the feet of the passenger to go all the length, but not play with them, since the hoses are nearby. Not ideal, but it does the trick during the winter.
Heater.jpg
Another option will be to see if one of the members in Australia can source you a used one over there, including the defroster chutes.
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