I have learned from previous threads on this site or the MBCA site that NAPA sells the block heater cord for a better price than you get from a Mercedes dealer.

I have a '98 e300 that has a block heater that has never been used. The owner's manual or other materials that came with the car when new stated that the block heater cord did not come with the car but would be furnished by the dealer free of charge if requested, that may explain the reference in your second paragraph.

There is a technical article on installing the block heater cord that might help you.

PeachPartsWiki: Block Heater Wiring