Originally Posted by tgompf47
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.
You guessed right -- it's a redundant chassis ground connection. If you reused the factory metal radio brackets, there will be another ground lug on the left bracket that you can attach the car's ground connector onto. Some aftermarket HUs also have that lug. Otherwise it isn't required (tape it back to prevent rattling).
See post 11 in this thread:
Stock head unit information -- pin-outs, subwoofer, aux inputs, and more