My OM603 have a block heater, but it is NOT from the factory, it is retrofitted when the car came to Norway in 1997. Euro spec mercedes never have block heaters from factory that I know of, but the dealers can supply it at extra cost.
Ditch the block heater and install
webasto in stead. No need for external power, need only 15 minutes for warm engine, or 20-40 minutes for warm engine + interior :-)