I don't know if you're over thinking it, but you're over fretting it. :)
It's a wiring harness. It goes from here |<----- to here ----->| It has a natural 'lie' to it, so it can't go anywhere else. Once you have the ends in place the rest is pretty much set. Along its path it has forks, detours, and dead ends. These also are pretty much set. You'll see they correspond with the reasonable locations of things like the horn, brake switches, alternator, etc. Most of these things are unique enough in location and terminal type to be easily sorted as you go along.
The trick is to get the harness securely fixed along its route such that it has no interference, interferes with nothing, has room to move where it has to, and won't get damaged. Have a lot of zip ties and accept that zip ties are expendable.
Have fun with it.
