Macbook only works in External Display mode (inverter not the issue)
I was given a broken Macbook 1.83 C2D with no background information other than that it only worked with an external display (no mirroring or extended desktop), iSight says it's not connected but appears in system profiler, that the sleep light stays on during use and it had a failing hard drive.
I turned on the machine, it flashed the gray bootup screen and then goes black. I connected it to the external monitor and it powers up fine and gets to the login screen. After logging in, I unplugged the display adapater and the video shows up on the macbook display VERY BRIEFLY, like for about 3 seconds before it goes to sleep. I could see that even with the brightness turned up it was very dim.
I replaced the lcd inverter board and installed a new hard drive. I boot up the machine and same problem, it only works with the external display connected. The dim LCD issue was resolved, however. When I unplug the display cable, the video switches to the macbook for 3 seconds and then goes to sleep. Press a few keys to wake it up and it stays on for another 3 seconds before going to sleep. Then, after about 3 times of doing this, it just stays in sleep mode and doesn't wake up.
What could be causing this? I thought it was the PRAM battery so i zapped it to no avail. Replaced the PRAM battery and the symptoms persisted. Is it a dying LCD screen? Are the display cables failing (which could explain the non-working iSight)?
I've tried to do as much research as I possibly could but yielded no results. Any suggestions, people? Thanks in advance!
Mac OS X (10.6.2)