Not sure exactly where the Block Heater would be located on a 300D but it will be in place of one of the 'Frost' Plugs on the side of the Engine Block.
On my SDL it is on the exhaust side about half-way along the block.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown