Why can't it use the headlight sense?

Argh, you're not listening!

Because the headlight sense doesn't know the difference between "off" and "on". It just knows "line is high" and "line is low" and adjusts the dimmer to two user-controlled presets accordingly. Different cars do it opposite of each other.

Okay, the above thing could be handled with an INI setting. But here is the other problem with using the empeg as your headlight-warner: Headlight warning is only useful if the key is out of the ignition. But if the key is pulled out of the ignition, then the empeg goes into sleep mode and the amplifier is turned off, hence no way for the empeg to make a warning sound.