I installed an amp and sub following the methods on the previously posted links. Everything you need for the amp turn -on lead and audio signal is on the connector for the sub. There's four wires on the sub harness. One (I think blue with red stripe) is a power for the stock sub, and you can splice in your amp turn-on lead there (most installers suggest using an fused lead, I didn't). I then took an old, mono RCA cable I had lying aroung, stripped the wires, and spliced the positive wire to the brown harness wire. I didn't do anything with the negative. I initially thought things would be screwy without a negative, but not so! You can then buy an RCA splitter to connect the mono RCA to the L and R inputs on your amp.
If you're getting some noise through the sub(s) from electrical interference through the car's chassis, your amp may not be able to use the low volt (2 volt) signal. If there's too much noise, you can try hooking up your audio signal into the post-amp connectors on the sub. There's three bolts holding the stock sub amp to the rear deck. Unscrew those and you'll see two wires from the amp assembly to the actual speaker. You can then tap into those for your signal. I think the larger connector is the positive, but test to make sure. Careful though using this method as it may be too much for the your aftermarket amp's inputs to handle.
I've had two different sub-amp combos in my speedie and I had to use the second method for one, and the first method for my current sub. The stock sub is still hooked up and works but is overshadowed by the two 10s in a bandpass box.