[BiturboS4] Mobile Speaker in door?

[Steven Lin]

The speaker, if you have one, should be connected to the DB-25 cable
in the trunk (or boot as you Brits call it).  IIRC, pin 19 is the +
terminal and pin 25 is - terminal.  Hook an audio source up and see
what happens.

Anyway, you probably should be wiring your handsfree kit via that
cable instead of via the head unit.  All of the needed connection
points are accessible through the cable: stereo mute, mic, switched
and unswitched power, and speaker.

--
Steven