I installed two 12inch subs with a 1000amp with the factory unit and it was quite simple to do.
Things you need: Line out converter, wire cutters, wire crimpers or sautering iron and a little technical know how.
You need a line out converter, they are really cheap and easy to use. You'll need to find the wiring diagram on the internet i had it but lost it. Just look its there i googled it.
1) First wire in your sub by running the the wires through the car and into the engine bay for the power I just found a ground in my trunk. Its a little bit of a pain if you have a sedan like myself.
2) You're gonna have to remove the molding around your factory radio unit. Be careful not to destroy the soft plastic around it by prying it.
3) Take the radio out by removing the supporting screws. now you're gonna unplug the unit and set it aside.
4) Their are gonna be a couple wiring harness, now one set is gonna have a bunch of colored wires in pairs. so that means ones ground and ones power. You're gonna need to find out which one is front left, front right, and back left and back right if you want the full range.
5) Now cut those wires and splice into them using another set of wires and bring them to the line out converter. Then find a nice mounting spot for the line out converter i put it in the glove box.
- Now this is really really brief and probably not the best explanation. If you need any help you can post on this or PM me. It took me a day to do and some help but its not the most difficult thing to do.