I can see by the first photo this was not caused by the switch. Any time you have a mechanical connection, meaning the contacts inside the switch, and the terminals in the plug that mate to the terminals in the switch, anything less than a perfect connection causes a little resistance, and current flowing through a resistance generates heat. That heat expands the terminals to create more resistance, and that makes more heat. When that starts with a pitted or arced contact in the switch, the heat migrates out to the terminals and makes them black. It also overheats the first four inches of those wires, but not to the point of melting the insulation. That part of the wire will just become real hard, and solder will not adhere to it, so that four inches must be replaced.
The additional clue is the total length of the wires has melted insulation. That means something on that line was shorted to ground. One real good suspect is if the truck has an aftermarket radio. The tail light circuit goes to the original radio to tell the display to dim at night. A different wire from the dash lights also goes to the radio to tell the display how much to dim. The clinker is there is no ground wire in the connectors for the radio. The ground is made through the braided strap bolted to the back of the radio. When people install an aftermarket radio, they often use an ohm meter to figure out which is the ground wire, and they mistakenly find the black/yellow because the meter reads through the tail light bulbs, then to ground. The same thing can happen with the orange wire at the radio, but that one goes through a fuse first, so it would just blow that dash light fuse. (Between the fuse and head light switch, that is a tan wire).
There is a 20-amp fuse in the pink supply wire, but if it did not blow, that means there was enough current flowing to overheat the wire eventually, but less than 20 amps. That is unless someone put in a larger fuse. If the black/yellow wire was used for the ground for an aftermarket radio, that circuit would have gone to ground through the radio's metal case, then through the outer shield on the antenna cable, and to ground through the base of the antenna into the fender. That is a dandy ground for shielding out interference, but it is a terrible ground for current flow. That could explain why not enough current flowed to blow the fuse.
Check fuse number five and be sure it is a 20-amp. If someone stuffed a larger one in there, and a proper 20-amp blows, that is a good thing. I have a trick for working with that.
Let me know what you find after jumping those two wire pairs. Be careful when you do that. If something is shorted, ten to fifteen amps through a stretched-out paper clip will turn it red-hot. It is best to not be holding onto it for very long that way.
Wednesday, January 2nd, 2019 AT 1:16 PM