Have you tried resetting PRAM ? (Booting with Cmd+option+P+R keys) Next step you can do is trying to reset SMC. (This progress is hard to explain so I refer you to apple's website for that Resetting SMC) Next up is finding the connectors on the logic board which lead to the backlight. If you see any corrosion/soldering damages, then u might have located the problem. I do not own a tech manual for mbp atm but I'm sure someone on this website can provide you with the location of the connectors on the board.