The engine wiring can degrade because the insulating material falls off leaving the bare wire exposed. This can cause many problems depending on which wires are exposed and what they touch.
Car Problem Reports
Mercedes-Benz SL600 Insulation Degrades on Engine Wiring Harness and Leaves Conductors Exposed
Mercedes-Benz SL600 Problem
Average Mileage: 78,500 mi (55,000 mi - 102,000 mi)
Visitor, 1994 Mercedes-Benz SL600, 102,000 mi
Electrical fire! Install new wiring harness.
Engine wiring harness failure.
Fried engine control computer.
No fix yet. Mercedes wanted $4,500 for an engine control module that was damaged as a result of the engine wiring harness failure. I found a computer rebuilder who quoted $495 for a guaranteed rebuilt computer (my own). Still looking for an engine wiring harness, since Mercedes is saying that could cost $1,500 or more, plus a couple of day's shop time.