You could inspire from my personal phone speaker I made.
This is very simple :
Take your hand free kit.
1- Cut the headphone wire and determine what are + and - wire.
2- Sold the + wire in serial with a 100uF capacitor.
3- Sold this to a RCA plug with some long wire.
It will allow you to easily place the Microphone in your car
Plug this RCA to your Aux In.
The sound is Mono. but you have sound in the 2 speakers. (I dont have rear speaker for now).
I plan to build a cradle for the phone that will make me connect phone on it and it's gonna be more simple.
I will try also to feed the mute wire with some hack from the phone.
Here is a starting link :
Gnokii