The main thing is that you install the circulating tank heater LOW in comparison to the engine. The heated coolant has to be able to flow up vertically via natural convection. Pretty much the lower you can mount it, the better it'll work.
Also, if you have it installed in series with your heater core, make sure your heat is set to on when you turn off the car. Most cars have a heater core bypass valve that closes the heater core coolant circuit if the heat isn't on.
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