Does copper wire go bad?

Copper wire will go bad. It will start at the connections and work it's way back into the wire. It will change green and black and will have high resistance. This is usually with fine stranded wire and not the bigger solid strand wire. Electricity flows on the outside of the wire and not thru the center so when it gets corroded and oxidized it will create resistance and make heat and the corrosion will spread faster into the wire. I had a bronco 2 that had badly corroded starter wires and it would start intermittently and took me a while to find the problem because it was inside the sheathing and you can't see it until you strip the wires insulation.