I'd start by removing and disassembling the charger receptacle. Make sure there's nothing in there that could potentially be causing a short. I don't know if that's super likely, but it's at least possible. And while you have the receptacle apart, replace that red wire with a new one that runs the full length that it needs to, without having to have the wire nut on there. Now that I think about it, the wire nut itself could be the cause of the problem: it's not exactly well-sealed, and it doesn't take a ton of corrosion to cause excess resistance, and therefore excess heat, which could easily have caused that wire to melt. If you want to simplify matters, just replace the whole receptacle, wires and all. Here's one on Amazon for a reasonable price. Best of luck!