You will have to replace the ignition control unit (after testing) that is located in the driver's fender behind the headlamp assy.
And be sure to check out Doc's quote. He is right. I assumed that the secondary would be in order. If not, start there. If you have done this, I would have the Primary side checked (I.E. the controller.)
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS