busted low beam on 2003 Mercedes CLK320 Halogen headlight

hello,  i have a Mercedes 2003 CLK320 coupe W209 model with a xenon halogen
headlight.  The low beam bulb is not working.  i replaced it with a new one as well as
using the other known good bulb and still does not work.  Could it be a ballast problem?   
How do i confirm this is the problem that needs replacing?   I understand you have to
remove the bumper just to access the headlight assembly.   thanks for your help.