HP DV8000 No Backlight w/new parts

I have a friends' DV8000 (XP Media Ed) with no backlight. The LCD screen works and displays images.

He brought it to a repair shop first and they ordered 4 inverter boards which he gave to me. Unfortunately, only one of those boards actually fit this model of laptop. I have ordered 2 others that do match with no joy.

Here's the long and short of it:

-->I have replaced the LCD cable--the WLAN indicator LED which is part of this cable assembly had been cut by the repair shop because apparently they couldn't figure out how to use needlenose pliers to work the LED out of its housing.
-->I have tried 3 inverter boards
-->I have tried a new backlight lamp (and even a 3rd lamp which isn't the right size, but at that point I just wanted to get light.
-->I have plugged the lamps into another laptop to verify that it lights
-->I have toggled the lid switch to verify that it does indeed go into standby as expected
-->It is fine when hooked up to an external monitor.
-->The dark screen symptom exists during the entire boot cycle: in other words, it doesn't just go dark when WinXP starts to load, but even the POST screen is dark and barely visible.

I'm hoping this isn't the motherboard. What else can I check. I have a multimeter but am unsure which pins/circuits to check. Any help will be greatly appreciated as school has started for this friend and he'd really like to be able to participate in his online class especially without having to borrow his mother's laptop.