Thanks bro.. I knew I could count on you but I went to a car and Stereo shop and they talking about 700 just to hook up what your talking about. Is tapping into the lines something I could do.You'll need a shallow-mount 8" because of that bracket. Not too many around, I think Pioneer and ID. If you will be doing a replacement, make sure it is a dual voice coil.
To be honest, with good aftermarket 6x9's back there the 8" may not actually be contributing much. You might consider tapping those lines into a monoblock amp and running something simple. I have a sealed 10" Infinity that makes quite a bit of base when measured on an RTA. So even that as an alternative will make bass you can actually hear.
What kind of Metra adapters do I use. ...Yes you can tap them easily. They are running open under the trunk silencer roof panel. You can tap them or get some metra adapters and run them to an amp that takes high/speaker level input. The battery in the trunk makes wiring easy. Infinity subs are around 60 bucks and a sealed 10" box will fit in the trunk easily. Some like to mount the amp on the box. I mounted mine to the seat back for easy access. Industrial Velcro will hold the box securely enough. Ported boxes are bigger and it might be hard to find one with the right dimensions. But it's might be worth it. Also, match your amp to the sub. Even 300W RMS can make a decent amount of bass.
You could have used these!