I got this out of the U13 Altima service manual, i assume its the same, i just did the probing like Jono stated, when i did mine.
Red = 12v
Blue = ACC
Brown/Red = Power antenna
Red/Blue and Red/Yellow = Illumination (for stock stereo, so comes from instrument lights)
Blue/Yellow = negative front left speaker
Blue/White = positive front left speaker
Brown/White = negative front right speaker
Brown = positive front right speaker
Green = negative rear left speaker
Red = positive rear left speaker
Pink = Negative rear right speaker
Blue = positive rear right speaker