Principles of Operation
WARNING: This page is about a different car, the 2007 Mercury Mariner and 2007 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
A fluctuating AC voltage audio signal is sent to the speakers directly from the audio unit to reproduce sound. The audio unit has a self-diagnostic mode that can be entered by pressing preset buttons 3 and 6 simultaneously.