As you see an image using a flashlight your focus needs to be within the backlight logic. I would do a careful inspection of both sides of the logic board looking for corrosion damage. Don't forget the displays cables and connectors.
You'll need trace the logic from the start on the logic board using a DVM and a set of schematics and boardview drawings to isolate out where the backlight power is lost as well as ID the damaged part.
This might be beyond your skill set