Don't use what you linked.
What you want to use is a Line Out Converter (commonly referred to as LOC). A LOC will have a high impedence (usually 10k or higher), and so can be wired in parallel to a load, also refered to as tapping a signal commonly, A stereo LOC is best so you can get bass signal from both right and left and sum it to mono for a mono sub install (which is the most common) basically you want bass whether from left or right.
The Amp in the rear of the car has front full signal for both right and left and is used for powering the door speakers. In this car it is considered the easiest place to wire a LOC.
see this thread for a lot of info on the stock amp, you need to know which wires are for input front right and left, it'll be 4 wires, and remote on is also useful.
http://www.ft86club.com/forums/showthread.php?t=42239
further in the thread it describes how to use metra harnesses to wire a LOC to the inputs without cutting any factory wires, costs a little more, but makes the work easier, and 100% reversible.