ribbon cable failure is a known issue with many G4 models. The ribbon cable runs through a slot in the lower case passing over (in many instances) the battery or some other internal part.
Typing and hitting the space bar bounces the top case. The slot then starts acting like a guillotine damaging, or a cam loosening, the ribbon cable from it's connector.
Partial fixes or work arounds involve moving or shielding the cable with an insulator; small pieces of paper work.
Sometimes reseating the cable at the trackpad end works (the connector is a pinch clamp) it lifts up to loosen, work the cable well back into the slot and lock the clamp by pushing it back down in place.
It's possible that a backlight LED or sensor has failed or been disconnected (especially if you've had the unit apart).
PRAM would contain the brightness setting of the backlight (I don't recall if default setting is 0 or 10) but otherwise should not affect it.
Remember to return and mark accepted the answer that best solves your problem.