Hello,
If you just want to install a subwoofer& it is relatively easy. Like previously menitoned, a line level converter is needed. The model I used is a SNI-35 and PAC (Pacific Audio Company) makes it and it is inexpensive. You want to put this device in the trunk and tap the signal off one of the rear deck speakers.
The sub is going to need to be in a box and the amp is going to need a wiring kit. You may be able to get away with 8 gauge depending on the power of your amp (more power means it needs thicker wire). Check the amp's manual it will normally say the minimum gauge required (remember lower gauge means thicker wire). The hardest part may be wiring the 12 volt turn on lead (needed if your amp doesn't have signal sensing) that tells the amp to turn on. I went with a JL 10w3v3 in a sealed box with a JL A1400 amp (500 watts) and it sounds great.
The only problem is that with the stock headunit, you will have no control over the sub. The sub volume will go up and down with the main stereo volume, but it is nice to be able to control the sub volume independently for a variety of reasons (i.e late at night, some songs have a lot/too much bass, sometimes you want to rock out w/lots of bass etc.)
A great device that allows you to do this is the JL Audio CL-RLC (google it). It can be used with any brand sub/amp combo that you choose. It comes with a volume knob that goes from the trunk (where the amp is) up to the driver. Mine is mounted low near where the drivers right knee would be. I can control the volume of the sub, from max to mute...the thing is great!
Hope this helps.