This project has turned out to be a big pain in the ass, but I am making some headway now. For those who come after me, take my advice: it's easier to build a loom from scratch than to modify a Euro one to match a US bike, unless you are willing to end up with a harness that doesn't match any published wiring diagram. My ADD won't allow that, so part of the work involved in my case is changing out some colors of wire that are different between Euro and US. But there are other things that just wire up very differently, and the Euro harness is not going to work for these. Differences I can remember off the top of my head:
* US rectifier takes two inputs from the stator, and has two outputs; Euro has one input and two outputs.
* US indicators and horn share a DC path; Euro indicators and brake switch share a DC path.
* US has a hand brake switch wired in parallel with the foot brake switch.
* US has rear indicators.
* US regulator and battery can both independently power all DC loads in the bike (I believe this is to comply with US law that taillight must illuminate even when the engine is not running), and thus are wired somewhat differently to Euro.
By comparing the old and new harnesses and their diagrams side by side, I was able to work out what changes I would need to make. Then I eventually found places where I could buy the ten colors of wire I needed mostly in small lengths to keep cost down. I then struggled for awhile actually getting started making changes to the new harness, before I realized it would be much simpler if I labeled every branch, so I could see at a glance the path each wire needed to take. I used masking tape for this, so I can remove it later.
Most recently I have been struggling to actually feed new wires through the loom. Pushing them through just does not work, the wires are not stiff enough to overcome the friction over the lengths required. Taping a gear cable to a wire and pulling it through didn't work, the two just separated. I was going to try to use a strand of steel wire and secure it more strongly to the end of the wire I want to pull. But then Ginch and his friend James stopped by yesterday to check on my progress, and James pointed out that at this point it would be easier for me to just pull all the sheathing off the new harness, lay out my new wires, and then slide the sheathing over everything (perhaps using a pull string around each bundle, and some grease to help things along). I'm going to try that later, I hope it will help.
I took lots of pictures of the hookups inside the headset and at the horn, but now that it's been a few weeks since I took everything apart, I am still a bit worried that I am going to have a hard time getting everything hooked back up correctly. But hopefully that will not be the case. I have to remind myself, it's just wires going from A to B, and I've built radio kits that have hundreds or even a thousand or more parts. I'll get this done.
For you Aussies, I bought some wire from https://jaydeeautocables.com.au
and some from http://www.rapidcables.com.au.
I also bought a kit of spade connectors on eBay because I only had one of the three sizes required on hand. I spent some time browsing https://www.ozautoelectrics.com
as well, they have tons of stuff, but in the end prices on what I needed were better at the other sites.
Hope to have a final update soon!