sexilexi05

I just installed a new stereo on my 2005 Lexus is300. Everything is working great with both the stock amp and my aftermarket one. Now my new head unit has a GPS system built into it. So I decided it's time for the old stock one to go. I got the center console apart to get those controls out, but when I took out the stock gps unit and replaced it with the shelf looking piece, now I have no sound through speakers. It's strange because when I plug the unit back in, I will suddenly have sound. While leaving the stereo playing music, I can unplug the stock gps unit and the sound will still play. I know I need to rewire something back to the stock amp. But how to do that or where to even begin I have no clue