A fuse holder with a digital read out. If you only want it to turn on when the car is on. Instead of wiring it to constant 12v, wire it to the ignition ( under steering coloum, possibly in a fuse box in the engine bay. Though, most of thoes types of fuse holders moniter the battery voltage and shut off if the same voltage is present for long periods... IE car is off and is not discharging. What is the make and model Ill do a little research. If you need help finding ignition in the engine bay ill walk you through that as well.