I've seen those $3 problems take a $1,000 worth of labor to figure out.
Sometimes elecrical grimlins can be hard to trace, especially bad grounds.
It is true that a $3 problem might cost $1000 in labor to fix but the problem is not that it actually takes $1000 of labor to locate but that $997 dollars of labor was wasted.
The problem is that most people take a very poor approach to troubleshooting problems, especially complex problems. I often see fairly knowledgeable people take a very non-systematic approach and waste many hours in addition to creating extra problems.
If a technician would just stop and think before doing anything when approaching the problem, they would greatly increase their probability of success and shorten the total time spent on the problem.
If a binary approach, (divide the problem in half), is used, most problems can be located in under 8 checks. That only takes a few minutes to do.
The problem is that most people approach the problem with the wrong mindset. In this case the OP already has in his mind that the problem is more complicated and troublesome to locate and repair than a complete re-wire.
The trouble with this approach is that you are embarking on a solution that might not resolve the underlying problem and more than likely add extra problems to the equation that you don't have initially. This only clouds and masks the underlying problem and consumes resources.
Unless the wiring harness is absolute trash as in having caught fire from a short or serious corrosion from being in the elements for years, then locating the underlying problem will be much quicker, easier, and cheaper.