I have a 2007 Prius. I also have the grill blocked, which I do every winter. I did notice that the engine cooling fans only work it the car is going 25 mph or under (I have a LED attached to the power input for each fan). I think Toyota assumes that air flow will not be needed because of road speed. However, if the AC turns on, the fans won't turn on and it may be needed. I don't really know as I don't have a wind speed meter in the grill not AC guages hooked up . Does anyone know how to disable the speed sensor from the cooling fans?
