Important to know is that the insulation that protects the wire from corrosion by resisting the passage or oxygen and moisture has a design life of only 20-50 years. For a mid-80's GM now in 2015, it's past half way to the "ideal" life-span, or 11-years overdue for failure, depending on care and usage (yes, mudding or fording rivers, or parking it in a field for ten years without moving it is not healthy for longevity).
You will see the mantra "check your grounds, clean any corrosion, test and repair your wires" as if it was an oft repeated prayer as a result
Modern cars with all of those fancy electronics are in for worse life expectancy, components like electrolytic capacitors have a design life of 10 years, and when they fail they can leak electrolyte that eats circuit boards causing the whole assembly to fail (usually at immense expense to replace). Ask me how I know