I have two MBP Unibody LED LCD laptops with no back light. I have replaced with known good display and cable and still no backlight. External works fine. Image is on LCD but darkly with no back light. I wish someone would come up with a more productive answer so I can move on with this problem. If we could get some kind of schematic we could test for proper power to LEDs . Note Remember, leds are Light Emitting Diodes. They only light up when current goes one way. If power goes the other way it won't light up.
Answer to: Back-light still doesn't work after a lcd panel replacement