Some low level converters for car audio (speaker level to line level) incorporates a remote out wire. One example is this one on ebay. On this page the functionality of such converters are explained like this:
When the converter receives an input with DC bias, it drives 12v to the remote output wire (blue). The B+ and ground wires must be connected for the remote output and the LED power indicator to work.
So my question is whether all car stereo audio outputs are DC biased and if this audio detection method can be used in all cars? (I want to use this method to turn on a device when the stereo is turned on)