If you mean a tank type heater(circulation heater) they have been put in parallel with the heater core and seem to work ok that way they do need to be mounted as low as possible.

I imagine you know where the freeze plug heater and radiator hose heater are placed.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.