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I guess I should clarify my statement some. I was referring to dash or instrument wiring. Sometimes you can save it. Engine harness is completely different. I always buy new. Rear tail light it the same as a dash harness. Sometimes it is ok.


I can understand your concern, of all the harnesses the most costly is the under dash. One thing that can make a difference in whether to replace is where the car came from a shore are,ie NJ, and/or if it was previously melted or modified. Even if not we have worked on cars (our own at times and for customers too) and they had hard start problems, burned up turn signal switches, etc.
Turned out the main harness had resistance and corrosion in the wires. Even if you clean the terminals there can be oxidation in the wires that creates resistance and causes heat, burned fuses, and melted wires. For the $500-$600 cost of the main harness it is not worth losing your car to fire. Or, having the car die on a trip due to an old harness.
Just for fun you can strip back one of the wires and see if it is green, if it is chances are the whole harness is a disaster waiting to happen.