These harness were made with bio-degradeable insulation and degrade by design..the problem is they did not calculate the ambient heat and humidity conditions correctly and the insulation degraded way before they wanted it to.
All of these degrade..... how long it takes is dependant on the conditions each is exposed to.
Every time I see the new "Green" Toyota commercial with their lapse time chassis returning back to Mother Earth I think of how far ahead Benz design was with their wire harness...trouble was they were TOO far ahead this time and their clock was running too fast....

I have a Tech friend who works on Jags of the same time vintage and he told me he has had to change whole chassis harnesses... terrible job.