unnown speaker wire

[JMW]

I know just short of nothing about sound systems. I am putting a new stereo in my f150. I have the 8 speaker wires for my four speakers but there is a ninth bare wire. What is it, where does it go? I would assume it's a ground but where do I connect it?

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[HardofWhoring]

A picture would help.
The color of the wire would help.
Do you have a diagram of either the head unit or the vehicle?
You didn't even say if the wire is coming off the head unit or the vehicle.