I just found out that the plug is sold separately by Mercedes and is called a Radiator fitting and the part number is 000 500 08 94. Price is $5.14 Canadian. Probably $2.99 USD.
Here is a pic of the original radiator that came on the car when it was new:
Here is the original part number:
Maybe someone can confirm this....My local Mercedes dealer says that the part number for the radiator above was replaced by this number:
A124 500 24 02. The original radiator may not have had the fitting that is broken on mine, which makes me think that I may have the newer radiator in my car. I don't have any pictures to verify. Does anyone know for sure?
Thank You