I may be wrong, but I'm fairly certain that the engine wiring harnesses on the M104 engines and the M119 engines (400E / E420 / 500E / E500) go bad for different reasons.
I have read enough posts from MB DOC and Benzmac about the insulation on the wires being changed and inherently defective on M104 engines only. I've even read the specific years (93-95) and VIN numbers in a post. There is an actual TSB regarding the insualtion problem on these M104 engines. I've seen pictures posted and seen actual harnesses in my mechanics shop. The insulation doesn't look like the one in the pictures on this post, nor on the picture that Steve Brotherton posted on the 95 E420. The M104 harnesses look like they are 2,000 years old, and the insulation has turned to dust and disintegrates in your hand. It's just gone. The problem on these M104 engines from 93-95 is systemic and all of them are either bad or going bad.
I've read enough posts to know that the harnesses do go bad on the M119 engines, but I don't believe it's the same. I think excess heat is degrading the insulation on the wires. If you go by the posts, it seems to me that a larger percentage of 500E's than 400E's go through wiring harnesses. 500E's generate more heat, thus more problems. Not all harnesses seem to go bad on these cars, too. I'm willing to bet it's a relatively small percentage. There is not even a TSB regarding an insulation problem on M119 engines.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".