This has been discussed at considerable length repeatedly since I've been here. It seems pretty obvious that the engine wiring harnesses are bad from 93-95, and affect mainly or exclusively different versions of the M104 engine. I think Benzmac or M.B. DOC posted a really good reply of which engines and chasis the bad harnesses come equipped on not too long ago.
Do a search on engine wiring harness and one or both of their member names and see what comes up.
It has been speculated by two of the moderators that the insulation on the wiring harnesses was altered to:
1. prevent rodents from eating the wires.
2. allow the wires to be more recycleable.
In any event, the insulation degrades in the car, and a whole bunch of terrible things happen, from making the car run bad, to unending check engine light problems, to ruining the computer.
In my opinion, and many others, they are all bad, and if yours hasn't gone bad yet, it will.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".