Oil leaks often contribute to the plastic wiring deterioration and often degrades coolant hoses as well. Obviously the oil leaks need to be repaired and the damaged wiring attended to. To pin point where oil is leaking from the engine may need to steam clean the engine first. Starting with a clean engine and then running the engine it may be easier to identify specific leaks. If the oil leak can not be identified, florescent dye may need to be added to the oil and the engine run for a while. After running the engine the dye used has florescence in the dye is visable under UV light and it helps pin points the source of the leak. Identify the color of the oil to differentiate whether the oil leak is steering fluid, brake fluid, or transmission fluid and try to identify the location of the fluid leak under the vehicle, ie front of engine compartment, rear of engine compartment , passenger or driver's side of vehicle.
This is a design flaw that was covered as a campaign. The engine design changed to the twin turbo because of this flaw.Cost about 15k if you do not have it done thru mercedes recall . They have to replace several computers. Happens around 75k on these cars.