A comment about the backlight enable signal: the logic board has test points for probing this signal. It may be difficult with the display installed, but you could use a multimeter to measure the voltage between ground and the backlight enable signal, which would give you a good indication of whether or not the backlight hardware is faulty, or if the computer just isn't enabling the backlight. You should be able to find more details about these test points in the factory service manual for the machine, which can be found with a quick google search.