1999 pontiac grand prix?

Leave your 6 X9's alone. You want them for the sound and high's. You'll need a separate amp for that 12's because the stock amp wont be powerful enough to push it. You'll also need to get the power wire ran from the battery to the amp. Thats a separate kit and it'll with the rca cables and so forth. Any regular stereo/audio shop can do this for you

No. If you have an aftermarket amp already, you can run your sub on that as well. But you may want to check into a crossover for your sub. It will provide more power, and less noise for your 6x9's. Max power, to the sub is what you want. More highs and mids from your other speakers. 3 way 6x9's are the best for this.

Dude, you don't need a twelve inch sub. The only thing your gonna be doing with it, is annoying the **** out of people like me who don't wanna hear that mind numbing stuff you degenerates call "Rap". People could care less about what your listening to when your cruising down the road. Keep your tasteless garbage away from tasteful ears. You want a twelve inch sub? Visit "Wrongway".

get a seperate amp for the sub. I don't know what amp you have, if it has enough juice or whatever.

I would just take it to an electronics store and have them do it.

Forget that, im 16 and i have a lamborghini, if you want pics of me and my lambo let me know