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Car Alarm, Car Stereo, Mobile Video, and Cruise Control Info for Installers, wealth of information on car audio
HOWEVER, your best bet is going to be find a local car stereo shop and at least go talk to them about what you want to do. at least to get a good idea of what you need to do.
also, you do not HAVE to change your factory radio to run amps and the like, however, alot of people do change them out as an aftermarket head unit will allow easier adjustment and tuning for your system.