The block heater fits in a plug on the right hand side of the engine beneath the intake manifold towards the front. I have never looked at the location directly, apart from seeing pictures on the diesel forum. My own cord for the heater just comes out of that side of the engine. If you do a search on ebay for a mercedes block heater, you will see pictures of what the heater looks like and the kind of cord that gets attached to it.

You may have a block heater installed with no cord attached to it. (a number of people have mentioned that condition on the diesel forum) or there may not be one there at all.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13