i got a 99 gs 300 and i'm trying to change the factory head unit but keep the factory amp and speakers. i can bypass the amp thats easy but how da hell do u keep the amp.. i got the wiring and behind the head unit there is a left and right input and also the amp turn on. i figured that would work but it didn't any help would help . thanks

 

Welcome to CL.
The first thing we tell people is that the info you are looking for is already here in the existing threads. Spend an hour or two using the search function with some good keywords, plenty of info on this topic.
What I will tell you directly is that if you have the standard Pioneer stock headunit, you're pretty much screwed. If you have Nakamichi, you can do it.