Audio Control Module (ACM)
WARNING: This page is about a different car, the 2009 Ford Mustang. However, it is still accessible from the selected car via links, so may be relevant.
The ACM can be powered up when the ignition is in the ON or ACC position. When on, the ACM directs audio signals to the speakers through separate positive and negative circuits for each of the audio channels. The ACM provides internal circuit protection for shorts to ground, shorts to voltage, or shorts between any output circuits.