You can DIY this in an afternoon. Get a red LED from any auto parts store. Connect it to the alternator and the positive battery terminal. It will illuminate w

You can DIY this in an afternoon. Get a red LED from any auto parts store. Connect it to the alternator and the positive battery terminal. It will illuminate when there is a voltage differential. Brighter is worse. Optionally add switch if you want to prevent full illumination when vehicle is off. Or add a relay spliced into the radio / ignition / accessories circuit.