Originally Posted by DangerMouse

Just to supplement other comments, the best sign of a failing wiring harness is "never been replaced, original equipment". In the early 1990's DaimlerBenz chose a supplier who used degradable harness components. To put it plainly, the wiring harnesses were designed to fail after an unspecified number of years in the hot engine compartment. The M119-laden cars are more prone to complete harness failure (for obvious thermal reasons).

It is a good idea to do this before you experience symptoms. One forum member had the harness in his 95 C280 short out the throttle actuator and control modules a couple years ago. Repair bill was in excess of $4k USD. Should be substantially less as a preventative maintenance item, especially if you do it yourself.

-DM