The only thing I could find in the manual was a torque setting for the plug. It is technically called a frost plug I think, or block heater plug.
Have a look at
http://www.defa.com/doc/200/218.pdf , the last page shows a diagram of the location, get into the defa website and you'll find a photo of it somewhere on a Benz or something.
I managed to get a socket and ratchet onto it - just... but it has not been removed yet.
Shine a pencil beam torch from inside the bonnet, from between the vac pump and the inner front corner of the airbox to the block area under the turbo - the plug is a gold colour.