That's weird...when I search on NAPA for neutral safety switch I get a little plug shaped thing with two bladed on it...looks nothing like my column switch
Have the backup lights ever worked since you put in the new wiring harness?
This may make things worse. But one of my schematics shows something strange that concerns the backup lights.
Check this out and see if you have anything like it.
View attachment 136779
You are looking for a wire labeled 18LG-24.
If you look close, you will see it in the number 24 position of the engine bulkhead fitting, going to that 4 sp backuplight switch I was talking about.
I don't think you have it there.
It also appears in the front lighting plug, number 24 position. But, it goes to another plug and then it seems to head back toward the back of the truck.
Not sure, I will try to check the other schematics.
But, if you have that other small plug, that might be where the problem is.
And, don't forget the plug under the back end where the lights split up to go to both sides of the truck.
You have been working around that on the tailgate problem and might have bumped that plug.