Doubtful that it is a "really rare problem", but I would have to agree with "hard to diagnose". Most cars that are driven have varying degrees of commonality with a purists values, and most of us that frequent this site are not "purists", but do understand & admire the dedication of those who are. I think the problem here is the difficulty of coming up with a diagnoses without having the car in front of you. There looks to be a lot of damage and degregation on your harness. Hopefully you have a good wiring diagram because you will need it. You will have to take this job 1 wire at a time, looking for damage, shorts to ground, etc. You will need to take resistance readings with your ohmeter to find the "weak links". It looks like an overwealming task, but if you take it one circuit at a time, you will find the faults and be able to repair them. There is no "magic bullet" answer to your problem, just many hours of studying the wiring diagrams, testing the cicuits and repairing the defects you find. Don't just settle for just getting it running, continue with freshening up your harness to avoid future problems.