First make sure you don't have any settings to dim the display. Check your system preferences and power settings.
Is the brightness at the highest setting now? Does it still not appear to be that bright as if it were new? If so your display lamps are near the end of their life. You'll need to get a new display unit. The florescent lamps used in the older macs wear out.
The other possibility is the LCD lamp power system on the main logic board (inverter) cable is loose or the inverter circuit has a bad solder joint or one of its components going south.
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Try resetting the cable and see if any corrosion on the solder joints. Cleaning might help. Did you get your system wet at some point?
Unless you are a very good micro-electonics solder thats as far as you should go. Over heating the components can kill them and the joints are very delicate.
It maybe time for a new laptop unless you can fine some used parts. Sorry ;-{