Some folks use a Thermo-cube in conjunction with a timer so the heater only runs right before you need the truck only when it's cold enough. Thermo-cube is installed after the timer. Some folks that have used a Thermo-cube under the hood have reported it doesn't run long enough, I believe it. I once took some temp measurements with a infrared temp gun under the hood on a nice even 0°F day after the heater had been running 4 hours, top of the valve cover 70°, upper radiator hose 120°, injection pump 55°, bottom of oil pan 40°, center of steering wheel 20°, truck had been parked over 24 hours. It all depends on how fast you need to start up, some folks (like firemen) don't want to wait for the heaters to start and like the underhood temps getting up there. I like the fact that with higher underhood temps you don't have to use the grid heaters and prolonging the wait to start system's/battery's life so that it does work when you aren't near a plug.