Since I dont know how the system is connected, I'm a bit afraid just to piggyback the wiring.

Before I changed the speakers, the music was muted when using the handsfree and came back again when you hang up, so some kind of connection exists. The two extra wires was going through the four pin plastic plug, inside the door, to separate connectors on the speakers and even on to separate connections to the speaker papercone!
Sorry I dont have any pics.

PeteM....how does your handsfree work, and where does the sound come from when using it? Any change after the BSW mod?