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 400E Insulation Degrades on Engine Wiring Harness and Leaves Conductors Exposed
Mercedes-Benz 400E Problem
Average Mileage: 181,000 mi (180,000 mi - 182,000 mi)