Ok. I can tell you that it can be done for sure.
The best thing to do here is to use a deck that has a fifth channel that is meant for the subwoofer. Then you just run your wire to the gain control, and from there to a mono amp and the sub in the trunk.
However, there is a much simpler way of doing this, without the need for a 5 channel deck. Doing it in this manner, you will inevitably sacrifice some performance. You could hook the sub in parallel with one of your back speakers. In other words - take the two wires that go to one of the back speakers, attach your sub wire to it, run it to the gain control, and then to the amp & sub. Note: this isn't the "proper" way of installing a sub, but I've done it in a '98 Malibu, and it worked just fine for me.
Oh, and don't forget the power cable for the amp, a fuse, and an on/off switch