There are stories on here about having a very difficult time removing the plug to install a block heater. When I considered this option years ago, I went with a lower radiator hose heater. I was happy with the choice. Not quite as a good as a block heater but perfectly adequate down to about 20 below zero in my experience.

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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