This applied to E32's produced after 1991 with high-end stereo featuring external amplifier mounted in the trunk. Earlier models did not have a dedicated connector, but a set of different connectors for speakers, power and so on.

The wiring for earlier models will be available here soon. 

Looking at the connector from the front, the side that plugs into your stereo, this is the wiring layout:

IMPORTANT! The wiring has now been decoded and the above table will be updated soon. 

Here's an image of an actual plug ;

The E32 is also often shipped with a celluar phone. Either one of the other wires could be a phone muting input for the head unit.

In the BMW wiring comments where Free has been used, it just indicates that my model didn´t have a original BMW wiring here at all since it came with a sound-system unit in the trunk that featured built-in amplifier.