Electrical

What's melting, the copper wires or the insulation? There is extremely little current going through those wires even if one were to be shorted to ground. Any chance some chemical got on them?

Any chance you can post a photo? I'm trying to imagine what kinds of things can happen on a five-year-old truck.

Wires typically break from flexing or from being tugged on. Constant tugging will usually pull the terminals out of the connector or the wires will break right next to the connector body. Flexing occurs from the engine rocking, which is normal, but that affects wires where they span between the engine and body. They can also rub through if the harness is laying on top of a sharp metal bracket.

The bottom line is the cause has to be mechanical in nature, not electrical. Too much current in a wire might overheat it but it will melt the insulation all along that wire, not just in one spot. It's rare for the wire to burn through. Only 5.0 volts feeds those sensors and if one becomes grounded, the Engine Computer will just shut the power supply down to protect it. There are internal resistors in the computer too that limit the maximum current to almost nothing. That means you do not have to be thinking about electrical problems. Now that the wires are repaired, be sure there is plenty of slack in them and they are anchored and not flexing. You're looking for a mechanical cause.