I have a 09 Impreza 2.5i I just replaced the stock headunit with an aftermarket one. On the stock one there was a blue/black connector wire that was just plugged into a metal piece on the back of the stock headunit. I believe its some kind of ground wire. When I put in the new headunit I did not reconnect this plug to anything, the new headunit still worked and played fine. My question is, does anyone know the purpose of this connector and is is necessary to have it connected anywhere. On a side note, the stock stereo also worked fine without this plug connected.