This look like a pain but not impossible
Ok. The best way you can go is to analyse each wire of this harness. In your post you told about you got the infos of wiring on Corral.net. But i'm not sure if you have a Haynes repair manual and a Volmeter. You need to get them if you want to resolve this problem. You can get them at canadian tire for a cheap price.
Haynes = 14$
volmeter = 15$ (but more you pay, Better using you gonna get in the futur. ex TPS setting.)
The first thing if this was me is to get another harness on a wreck mustang and trying to set them in serial with some tap- in connectors. And tring the other one with your headlamp switch. If you find that the problem is not on the harness. You just have to remove the tap-in connector and seal with some tape. Like this you let the original harness at his place and you don't have to cut wires.
If the problem is more important.
You have to search in the haynes which wire could be problematic. The headlamp switch is at the page 12-37.
Hope this help.
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