Does anyone know a website that can help me out with this?
i would also like to know an answer to this...cmon guys....
Use the search button guys, it's all been covered before. Have a look through the Car Audio section.
Couple of ways.
You can have your existing unit modified to have pre-outs, we have a forum sponsor ASR audio ASR Audio & Electronics is their sub forum. depending on your location, they may/may not be able to assist you. He does fantastic work and comes highly reccomended by anyone who has used his services.
You can use a line out convertor (LOC) which converts your speaker wire into a low level signal.
Or, you can use an external amplifier which has high level speaker inputs. You use the signal off your speakers wire as the amps audio input, then adjust your filters to suit