Ok, basically our cars have 2 antennas connected to the stock radio, one is the power antenna and another one embedded in the back window glass. The 2 antennas are connect to the back of the stock radio with 2 separate connector. A large circular connector for the power antanna and a smaller one for the window antenna. Here's a picture on Koji_H's
Legend Tuner
Now for the many of us that replaced the stock radio with a aftermarket deck. Aftermarket deck only comes with one antenna connector, it's the larger one for the power antenna, so most of us just leave the window antenna disconnected.
But then many of us also tend to break that power antenna easily. So instead of replacing the power antenna, we could just hook up the window antenna which will work almost equally well. But to make the window antenna connector fit the back of an aftermarket radio, you'll need that part blo described in the top of this post.
Hope this helps!
Hey Ian, I need to hit up that autozone too, I bought mine for $11 at circuit city, I'm gonna return mine and buy another one at autozone. Thanks