OK, I looked at the WDS.

The black plug is connector X18774
It is an 8-pin (according to WDS!) black connector, and connects to the subwoofer speaker, mounted underneath the rear parcel tray.

The wires are:

Pin 2 = Blue/White = Sub 1 pos
Pin 3 = Blue/brown = Sub 1 neg

Pin 4 = Yellow/Black = Sub 2 pos
Pin 5 = Yellow/Grey = Sub 2 neg

Pin 8 = Yellow/Red = Sub 3 pos
Pin 9 = Grey/Brown = Sub 3 neg

Pin 10 = Yellow/White = Sub 4 pos
Pin 11 = Yellow/Brown = Sub 4 neg


The clear connector is X400, and is the connector for the telephone loom. Telephone is optional, so if you have to phone fitted, this connector is not used.
It contains battery power, ign power, ibus, illumination, ground, telephone mute, telephone audio out, and telephone on wires.

It is the most convienient place in your car to connect my NavCoder software :-)