I was able to hack my 98 Corolla's main unit to add an aux input for my MP3 player. It had a cd changer whose audio was hijacked. So to hear the mp3 sound I plug in the mp3 player into a 1/8" stero jack which switches the audio input from the cd player to the mp3 player. Note the cd player needs to keep playing so that the main unit is fooled.
I want to do the same for my 2000 Chevy Impala. HAs anyone tried it? I have the factory main unit with FM/Tape/CD player. Does the CD player have an output to the main unit? I will open it tomorrow & find out.