Any and all advice on the below scheme appreciated

I have a Samsung Galaxy S2.  I want to hack into the headphone and microphone leads to be able to contrive a hands free set up via my car audio.  The phone is normally in a cradle and out-putting music to the head unit via a lead between the 3.5mm sockets on the phone and head unit.  I noticed when I get a call, the phone stops playing music.

I would ideally wire the headphone leads to a 3.5mm stereo plug, which will plug into the head unit and connect the micro phone lead to a micro-phone for a defunct Motorola hands-free kit, that is still installed in the car.  I would have to connect a 3.5mm plug or socket to the microphone lead, but I can do this bit myself.

I am a little worried the Motorola microphone will have more impedance or whatever than the tiny unit built into the GS2 headphone set.

This may seem like a roundabout route, but I have tried a few alternatives already ...

I started off trying to use the phone's handsfree function and inbuilt microphone.  This doesn't work, no-one can hear me.

I tried a mate's bluetooth kit that clips onto to the visor.  Ths worked well but Bluetooth drains the phone battery very quickly, and the cradle doesn't allow me to connect a charger .... hence the idea I am seeking advice on.

Has any one seen a set of leads that will allow this or can anyone tell me of an outfit that could wire this up for me?  I am not good enough to do the soldering myself.