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.

Check out the attached picture that shows an "easy" place to inspect the wiring harness. It's NOT the best place but is easily done and will give you an real-world check to see if the wires are bad or starting to go bad.

Remove the front facade by lifting up and locate the farthest sensor on the right.

If it's 'bad' there, then it is in worse condition at hotter places. It needs to be replaced.

That sensor was the first place I noticed that the insulation on the wires was deteriorating. I think the insulation on the wires there is probably a fair and accurate representation of what you could expect for the rest of the wiring harness.

Amen, and that should settle that.