I believe your model still incorporates the ballast resistor...if you still use series glow plugs,you need it,if you`ve upgraded toparallel glow plugs,you do not-you put the 50A box in its place...The manuals recommend minimum 6sqare mm. thick wire between the plugs,however I `ve noticed the wire coming to the plug of cylinder n-r 4 is not that thick(in either case-parallel/series)...maybe 11awg. I think 10AWG wouldn`t hurt anything...
Some more info:
http://en.wikipedia.org/wiki/American_wire_gauge
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm