This company also provide some "useful" kit AND advice :
Sio Communications
I wanted to connect my Nokia Car Kit 126 up to the speakers in my Volvo S60 - the Volvo dealer said the only way it could be done was by purchasing THEIR interface box which also required a software download into the car systems before it would work - at a MEGA cost !
Sio Communications provided me with an interface cable, full instructions and FREE telephone support at a fraction of the cost -I can highly recommend them ( even though I don't get commission ! )
In use the phone audio is routed to the front speakers of the car stereo, whetehr the stereo is on or off, while the output from the radio / CD etc ( if it is turned on ) is muted during the phone call, then reinstated when the call is completed.
The web site is worth a look at even if you buy elsewhere as there is some useful stuff including wiring information etc.
Edited to add something I forgot !