There's an op-amp driving a GPIO pin on the CPU. The actual voltage at which it flips is measured relative to the processor's 3V supply line: this is set to be roughly 3/5 of your car battery voltage.

Assuming your battery is healthy and being charged, it's voltage should be about 14.4V. Off charge, it should be about 12.6V.

Hence, if your engine is off and your battery is good, you should only see the icon come on if the battery is loaded (fan? amplifier?) sufficiently to bring the voltage down to 7.6V. I would say, if you can do that to a battery then either the battery is sick, your wiring between the battery is causing too big a voltage drop or your amp is TOO B***** BIG