The one block heater I have seen in a Volvo engine was installed in place of
one of the cylinder soft plugs down on the side of the block and used what the
old TV repairmen called a "suicide cord" for power. It has a rubber seal
and a bolt through it that expands the seal in the hole and a tubular element
a bit like a very short piece of an electric charcoal starter.
If I remember right it was installed on the right side near the oil filter,
probably in #3 facing forward. You'd probably have to pull the manifolds to
install it.
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George Downs Bartlesville, Heart of the USA!