sound system wiring on a '98 lumina

In my '98 lumina I still have factory speakers installed. They still work, but I want to wire a sub box in place of the rear speakers. I looked at the speaker wires to find the + and -, but they were not labeled that simply. On the left speaker, there is a brown wire labeled A and a yellow wire labeled B. On the right speaker, there is a light blue wire labeled A and a dark blue wire labeled B. Does anyone know which wires go where? Also, I know some subs require power, I need to know where i can take the power from. There only seems to be the power line for the trunk light, and tapping into that doesn't seem like there would be enough voltage. Any help is appreciated.

Terminal A is the return and terminal B is the signal feed. As far as power goes, tap off the 12v power drop under the glove box. Run some juice to the trunk, and boom, there you have it.

I would run a cable directly from the battery to power an amp that would drive a sub. and I'd recommend using (depending on the size of the amp) at least a 10 gauge wire, preferrably an 8, (or even a 4 if you're talkin' LOTS of watts) and have a fuseable link, too.