Owners manuals are not always right about standard equipment, *but* if your engine is like my 1984 300SD, the extension cord has two round female holes in a rectangular plug. It's similar to the "cheater cords" that TV technicians use to diagnose TV's with the back cover off.
Not familiar with your engine, but mine is on the passenger side, maybe halfway down, if I remember right. The cord gets routed and ty-wrapped to the front of the radiator so the cord can come out of the towing hook cover in the front bumper. The block heater replaces one of the freeze-out plugs.
Hope this helps,
Robert W. Roe
1984 300SD 173K mi