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 S320 Insulation Degrades on Engine Wiring Harness and Leaves Conductors Exposed
Mercedes-Benz S320 Problem
Average Mileage: 83,666 mi (60,000 mi - 111,000 mi)