As a side note to the setting in emplode:
I had to put a pull-down resistor between the mute line and ground to make the muting work in my car. It basically looked like this afterwards:

ground ------------+-----------

                   |

               resistor

                   |

mute line ---------+----------- empeg mute input

with a 10kOhm resistor, it worked perfectly (even though a 1kOhm resistor did also work, I tend to use as big resistors as possible for pullup/pulldown purposes, to minimize the current drain they produce).
Edit:If your handsfree set does a pull to ground to do the muting, you need to replace the "pulldown to ground" resistor with a "pullup to 12V" resistor. In other words, just take the above diagram and replace "ground" with "12V" and you are doing the right thing.

cu,
sven