So I just bought an aftermarket sony stereo for my 3 . It looks ok but does not fit as even as the factory of course and its not a 6disk. Does anyone know about how much it would cost to have my factory hooked up to my subs and amp? I do not want to do it myself...Ill mess something up. LOL ..

 

A friend of mine had a truck done at best buy or something for something around $100-200 and had more done than just tapping into the radio harness. Honestly that's a way easy job to do if you're just adding on a sub. The amplifier may have a high level input or a speaker level to RCA converter, and that has wiring information with them on what speaker wires to attach to.
If you have a 2000 watt stereo, 12" subs and the rest it should be an easy addition to wire into the stock stereo.