The easiest way to tell if the backlight is an issue is this:
Turn off the backlights altogether and compare a known-good notebook to a faulty one. You may need a good lamp or sunlight to see what's on the unlit screens. If one is still obviously having problems, you can rule out the backlight.
I would not be surprised if the fault lay in the circuit that drives the LCD. On the older Apple Powerbooks (which I've spent time piecing together), this is a separate board which also contains the brightness/contrast controls.
In my limited experience, where an LCD actually fails, it's due to physical damage, else one of the integral driver ICs has failed, in which case you get a blank vertical bar of dead pixels, terminating at the center of the screen, and it doesn't sound to me like that's what you've got. In instances where I've noticed problems similar to what you describe, swapping the LCD made no diff.
On 19 Feb 2004 08:13:22 -0800