The arduino will be powered with 12V switched power from the car. Most versions of the arduino have a power regulator circuit built into them, so it can easily accept 12V input. No filters (capacitors) needed as they're built into the regulator circuit.
Quite true, it would not be too hard to program the servo to open incrementally. However, I don't see much advantage? As I see it, ideal operation is to wait until just before the electric fan turns on, then open the grill block. This ensures the radiator is completely full of the hottest coolant possible. That means our bang for the buck of airflow will be the greatest. I'll drop the coolant temp down to just above where the thermostat closes, and then close the grill again. It will cycle open and closed. I don't think it'll be too often, but testing will tell. If it is actuating too often, adding variable grill block logic would probably be advantageous. If anyone sees flaws in my logic, do tell!
