Doing it your self, unless you are an electricky guru, is a brave project even for the most seasoned home mechanics. About few hundred dollars and much much pain if doing it on your own.

A painless wiring system can lessen the mental anguish a little and can be found on the net eaisly with a simple search. They are about $700-800 Aus for a simple kit.

Or you can pay a sparky to do it. It then comes with a warrenty, no mental anguish, no hours lost in the shed, but at a significant cost. They cost heaps cause its a complicated process to get it right. From memory, my Dad had his old hilux quoted to be rewired for about $1500 - $2000 Aus, so its an expensive exercise. I am sure that one of the auto sparkies on this site should be able to give you a much better figure than I can.

Good luck.