you can run a fan switch in your top house i believe you can get one from kenlowe.i have used one before when putting an electric fan on an anglia,it worked good for a while well maybe a year and started to give trouble,i just replaced it and all was good again,no idea how long next one would last i sold car not long after.
its just a tube with a sensor on a thermo switch you can adjust to suit what temp or when you want fan to cut in.
I'm sure they are available in nz somewhere.