Pull the breather cover off of the engine and look at the wires - the insulation on the wires will be cracked and exposing copper if the harness is bad. The problem is simple - wires short out to one another and create false readings in sensors or wrong actuations. It can create a multitude of different problems depending on what's touching what. For instance, we had a '94 and a '95 E320 (both with the old M104 inline six) in recently. Both of them had misfiring coil packs and check engine lights and other problems (with variable valve timing units, air pump clutches, etc.) due to bad wiring harnesses. Replaced both wiring harnesses and the cars were back to flawless functioning of the engine.