I no longer had Bose. Does anyone here have pictures from the back of Bose HU cuz I have a one mysterious plug not being used? So, I don't know if it was something left over excessively when converting from Bose to an after market Pioneer.

 

Hmmm. There should be two plugs. Larger one is for power/ground/front speakers signal and the second (smaller) is for bose amps on/off and rear speakers signal.