Hello CG,
I know you are no rookie when it comes to working on riders, but wiring repair is seldom a specialty that many of us are good at.
First, one would think that the 20A main fuse would fail and shut everything down, but new equipment is not designed as well or with has much care.
If it mostly involved the head light wiring, that should not be too bad. Melted insulation dripped all around can make things look much worse than they really are.
I've been searching around and so far, I have not found a printed diagram.
Equipment manufacturers have gotten very good a hiding or not provide service information like wiring diagrams. They are only interested in more profits from selling new machines. The fight over "Right to Repair Laws" has made many just quit providing any service data except to their exclusive "network" of dealers. The problem is.... MTD has no dealer network.
On with the problem:
I have been in this scenario quite a few times in both the automotive and the small equipment side of things. Rider mower harness is much easier than inside a vehicle dashboard.
Look at it objectively. The insulation burns off.... but the actual copper wire strands do not usually burn hot enough to break open. There are also usually short, unburned segments that will still have insulation to determine what color.
Make a list of wire colors as you inspect the entire harness. White, Red, Yellow, Light Blue and light or dark Green are the most common colors.
Separate one complete wire at a time using a knife or razor blade out of the bundle of melted insulation. While doing so use a blank piece paper to draw yourself a crude diagram. A diagram will be your road map has you go through and replace one wire one wire at a time.
As for replacement wire quality, only buy the wire from a parts store like NAPA or rummage through any scrap machines that you might have. DO NOT BUY WIRING ONLINE! The online "universal" rolls of wire are mostly Chinese or Asian copper clad aluminum. The aluminum core rots if exposed to any moisture.
