I don't know about the specifics of that year, but in general when adding an amp and sub to a stock head unit, you'll need to tap into at least one pair of speaker outputs and feed that to a line output converter which then goes to your amp.
Some amps have speaker inputs built in, but the general consensus seems to be that these integrated inputs are only marginally better than a cheapo passive line output converter, and an active converter, such as an AudioControl LC2i, is a much better option.
As far as tapping into the speaker outputs, that can be either easy or hard depending on your skill level. I generally don't recommend cutting up the factory wiring unless there's no other choice, and absolutely don't recommend using wire tap connectors! What I've always done is purchased a regular harness intended for installing an aftermarket car stereo, and a "reverse" harness, which is intended for repairing a damaged stock harness. Connect the two together and you have an inline connection that you can splice into that isn't part of the actual vehicle and can be removed if needed.