I want to put one Alpine 12" in. but I want to keep factory head unit, if possible. What will I need?

 

I think the stock units have a sub input on the back of it, at least my stock unit did in my 850, so if you used the stock head unit, you would need the audio cable for the subwoofer, and an amp, and enough wire to run from the amp to the car battery, and wire for the ground, and wire to hook up the amp to the subwoofer. It's pretty straightforward.