oooooohhh, you have a std cab...I was thinking you had a runner. Ooops...I don't know what you have for wiring around and such, but when I do an older truck I just run new wires for everything. I would def. tape off that second black wire for now, unless someone wired the speaker negatives back to that single black ground wire. That could be why the different colors on the other end. Another reason to just run new speaker wires.
