To get rego in WA you need to run factory ecu and retain all the sensors and pollution equipment etc. It is a massive job to make a custom loom. I used the engine bay harness and then chopped up the other half of the harness removing all the stuff not required and combined the 1200 loom into this. Bits needed to be extended etc. But it still took a considerable amount of time.

Even with my experience I would not start a loom from scratch. (p.s. I have done more than just teaching)