Backlight notebook doesn't go on, any suggestions?

Hello everyone,

I have a notebook and its backlight usually doesn't go on. "Usually", because the backlight does go on during the POST screen (consistent), and occasionally when I take the notebook out of stand-by. In the BIOS setup screen the backlight doesn't go on either.

In the name of science, I've attached the cable that goes from the inverter to the screen, to the screen of an other notebook.

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When I do this, the backlight of the other notebook does go on, and the screen turn white. The backlight remains on.

I came up with a number of things:
Maybe the backlight of the notebook is defect (even though it does go on under certain circumstances).
Maybe the notebook detects that the lid is closed while it isn't, and it turns off the backlight because of that.

Could someone maybe add or remove a theory? Any thoughts? Thanks in advance.