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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)

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As it says at your link, this is intended for bridged outputs. That means that each wire of the speaker output has a DC bias relatively to vehicle ground. However it does not mean that there is a DC bias across the speaker wires - the latter should be avoided, and an amplifier topology that produced it would need to have a DC blocking capacitor before feeding a typical speaker. An example might be an amplifier which only drove one wire, and had the other connected to vehicle ground - there the DC bias from the unipolar vehicle supply would need to be removed before feeding a speaker coil.

If your amplifier is not bridged, or for some other reason has a DC blocking capacitor, or does not put the expected DC bias on the speaker leads, then this detection scheme will not work.

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  • Not all car stereo outputs are 12VDC BTL where you can expect about 6VDC wrt ground on each speaker terminal .The voltage across the speaker is supposed to be zero .It is generally less than 100mV .In a previous life I detected low level AC on the speaker outputs to assure greater compatability with more car stereos .
    – Autistic
    Jan 2 '17 at 2:58

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