i have a pontiac sunfire 2000, i dont know if putting a system will mess up my car.
If it's done right, it won't mess up anything in your car.
The people who say it won't be as loud or sound as good as hooking it up to an after-market head unit are wrong. A 200 watt amp (for example) doesn't suddenly become a 100-watt amp because you're using a factory deck for a source; in fact, I've found that I usually have to adjust the gains down more when connecting to a factory radio than I do when connecting to an after-market radio. And the sound quality will be fine if you don't use a low-quality line-out converter. I've installed literally hundreds of amplifiers with stock radios, and in almost every case you wouldn't be able to tell the difference from using an after-market deck.
The line-out converter is the device that is connected to the speaker wires and converts the sound to a lower-level output that can be used with an after-market amplifier's inputs. Here's a very good one for a good price (scroll down to the NE-7V): http://www.autosound2000.com/adaptor_products.htm
The other tricky part in your car is telling the amp when to switch on and off. Your factory radio is controlled by the computer data bus, and you can't tap into that wire to switch the amp on and off when the radio turns on and off. Instead you have to connect the amp's control wire to a power source that will turn on and off with your ignition key. It's not too tough if you know how to test the wires, but if you're unsure it's probably a good idea to have a professional installer do the work for you.
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