I was trying to find out what this wire does.  I know it says "Mute" but is it an input the makes the unit mute or is it an output that mutes other things when the unit is muted?  If it is a control, is it actuated with a High(12V) signal or a low (GND) signal.

One of the things I miss is a single button mute.  My car does not have built in controls for the sound system and I am looking for a way to make this happen.

usually for things like this, a DMM helps, testing the wire voltage when pressing MUTE, if theres no signal change, it's an INPUT, if theres a change, it's an OUTPUT

usually, its also in the installation manual, that most people tend not to read.