+1 on fixing the wires. If you're willing to put the time in you can get decent quality wire and just remove one stand at a time from the main harness and replace it. Depending on how good you are at soldering you can actually carefully replace damaged strands and heat shrink over the quality solder joints. If you want real high quality results you can crimp a non insulated connector over wire joints and fill the connector with solder. Zinc-aluminum solder works best for corrosion resistance but you can also use rosin core solder and use weather proof heat shrink. The biggest things to identify would be brittle wiring, crumbling insulation, and lack of continuity. Speaking of which a high quality auto ranging multimeter would be crucial for measuring resistance across points in the harness to check for loose plugs, cold solder joints etc. Search for some of the posts on wiring by drtweak, hvyman, poodles, and especially jetjock.
