Well if the GPU is fine (since it works on an external) then it's the LCD or Inverter. And as you already replaced that, you can get a new inverter [http://www.ifixit.com/PowerBook-Parts/G4-Aluminum-15-Inch-Display-Inverter/IF154-038|here]. To confirm you need an inverter shine a flashlight and see if you can see anything on the screen. If not you may need replace the [http://www.ifixit.com/PowerBook-Parts/G4-Aluminum-15-Inch-1-1-25-1-33-1-5-GHz-Display-Data-Cable/IF154-045|display data cable]. Sometimes the cable's just loose, but since you replaced the display, you would have reconnected it if it was.