I have a '99 LS with stock navigation and a stock CD changer in the dash. I am fairly knowledgeable about stereo systems in cars, but I am a little unsure about installation in this vehicle. What is the best way to install an aftermarket subwoofer and amp while using the stock head unit/changer? I've heard things about using a line out converter from the changer. Any thoughts? I just want to do this the best way possible! Please help!!