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 300SD Insulation Degrades on Engine Wiring Harness and Leaves Conductors Exposed
Mercedes-Benz 300SD Problem
Average Mileage: 227,250 mi (197,500 mi - 257,000 mi)