You're getting light, so your backlight is working. The led's behind the keyboard are wired in series, so the circuit itself seems to be fine.
So it would seem that the thin sheet of polycarbonate that diffuses the light and spreads it across the whole keyboard has either gotten misaligned with the side-emitting led's, or, something is blocking the light from the led's from entering the medium.
You could try turning your laptop upside down and flushing whatever debris might be stuck under the keyboard with a can of compressed air. (Though I can't imagine how it would have gotten there in the first place)
If a liquid spill is the cause, it would have to be some kind of opaque liquid (milk, coffee, etc.), and you could try to take the whole backlighting sheet-assembly off the keyboard. It is attached with adhesive. Be warned that you can very easily snap one of the smd led's that are on the circuit in the middle, which would destroy the whole thing in the process. They are very fragile. If you can manage to get the backlight assembly off, you can just wash it in running water, as long as you wait for it to dry completely before reassembly. This will activate several water damage indicators that are on the reflector sheet, though It isn't really an issue for a model this old, unless it is part of an enterprise/business program.
Easiest thing to do would be to just get a new keyboard backlight assembly from amazon/ebay/wherever for about 15 bucks. You will still have to gut the top case almost completely to make the replacement, but it's a whole lot easier than trying to clean the old assembly without snapping one of the led's.