Well, today is that day. It's one of the thing I detest the most....electrical wiring. An old mechanic I used to work with years ago told me, "If you hate doing something, it's because you don't understand it well enough." That did ring true with me, so I spent a significant amount of time watching the guys that were good at electrical and asking questions, until the mystery went away.
I am a firm believer in new wiring in ALL my projects. I hate wiring and I hate troubleshooting electrical problems, so I always install a new harness. I'm not a muscle car purist, I'm a hot rodder. If I see something that can be improved on, I do it.
Plus, it's insurance. I would hate to spend $10-20K on a build only to watch it burn to the ground, due to cheaping out on wiring.
I started tying up the harness with a few clamps, while I decided what needs to go where. Don't be afraid to cut tie wraps after you have already routed your harness. Make it flow. Nothing is more frustrating than having to work under the dash that looks like a birds nest.
To keep things simple, I opted to buy a headlight switch that matches their terminated plug, rather than rewire mine to match it. After a quick phone call, the tech guys gave me several part numbers that would cross to a later model headlight switch. Way easier and I was able to reuse my existing bezels.