Unless there has been significant modification done to the harness there is no need to replace it. Usually trouble is caused by, in order:
1 -- Modifications done by someone who has little experience with wiring.
2 -- Switches.
3 -- Connectors.
4 -- And last the harness chafing against something on the frame causing damage to wires.
1 -- A process called "returning it to zero". In other words removing any modifications done, repairing any wires compromised during the modifications, and making sure all repairs are sufficiently insulated to prevent problems in the future.
2 -- Using WD-40, with the key off spray liberally into a switch while operating it. This flushes out most contamination and lubricates the mechanism. I do this as part of an oil change, and any time it has been left out in / ridden in the rain for any significant period. 70,000 + miles and no problems with any switches. Pay particular attention to the function of the brake light switches: they are prone to early failure and the last thing you want is to get rear ended by a car because of a defective switch!
3 -- Water migrating into connectors will cause corrosion and eventual loss of function. To prevent this obtain a tube of dielectric grease from practically any auto parts store and apply it as follows: unplug connector blocks, clean out any obvious dirt and / or water from both sides, squeeze a liberal amount of grease into the socket well, and plug it back together. The grease will extrude through the connector and make a mess which you can wipe off as soon as it's locked together. This absolutely excludes water and prevents corrosion. Don't worry -- it will not interfere with making a proper connection in any way. This can also be applied to battery connections, fuses, and the ground lugs bolted to the frame. Pay particular attention to high current connectors at the starter relay, rectifier / regulator, and ignition switch. About the only place it can't be used is on light bulb bases: the heat from the bulb will eventually break down the grease. Like this:BWD CL75 - Tool / Chemical | O'Reilly Auto Parts
4 -- Inspect the harness for proximity to any sharp objects (Unusual for a completely stock system, but if modifications have been done you never know!) and move the harness away from it or wrap it to protect it.
It's not as bad as it sounds: a couple of evenings work should be sufficient to prevent further problems.