As shown in the post regarding the pinout of the external amp DIN connector, there are six signals:
1) Ground - also connected to the chassis of the unit.
2) Remote Turn-On Signal - 12V or 0V, I am not certain which voltage correlates to an 'on' signal, but I would assume that when 12V is present, the amp is signaled to turn on.
3) Four audio signals (FR, FL, RR, RL). I believe these are tapped directly off of the preamp board (wherever that is inside the unit), and do NOT have +/- signals. These are line-level audio signals.