Check the wiring harness by cutting it open in the bend where it plugs into the coils.See if there is still insulation on the wires.Be careful not to cut the wires.
Also check the resistor tips (long rubber pieces between coil and plug) to see if the ends at the plugs are bad or have a green dust like buildup on them.If they do,replace them.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."