http://www.sau.com.au/forums/topic/146746-r32-gtr-4wd-to-2wd-switch/
A tutorial
http://www.sau.com.au/forums/topic/327206-fitting-a-4wd-rwd-toggle-switch-to-an-r32/
As for amperage of the wire if it is 30A fuse then go get 35/40A wire (I have some at home at it is not that thick)
If it is 10A then use 15/20A wire. Simples
A fuse does not blow at its exzact rating. It will blow above the rating so if you leave some room for the wire to handle the extra current then you will be a ok.