ford ranger headlight switch wiring problem

The terminals typically overheat due to a poor connection or an arced contact inside the switch. If just the connector body is melted, it was usually due to that terminal. If the switch is the culprit, you'll find the area around the rivet that holds the terminal to it is black and crumbling. The switch usually affects two wires and two terminals. Either way the switch should be replaced, and the melted part of the connector should be cut away, then the one or two affected wires can be attached independently. Use regular crimp-on terminals, but solder them also to prevent a repeat problem.

You will also find those wires feel hard for about the first four inches. That is from being overheated. You won't be able to flow solder on that hardened part and it will always make a poor connection so cut that part away. Splice in a new piece of wire the same gauge, be sure no sharp points of wire ends are sticking up to pierce the heat-shrink tubing, then seal the joint with that tubing. Never rely on electrical tape. It will unravel into a gooey mess on a hot day. Crimp AND solder the terminal to the new piece of wire and plug it into the switch.