m17 R5, screen backlight not working

My M17 R5's screen backlight has quit working, and I'm reasonably certain it's a hardware issue since the problem began after replacing the cooling fans. I've tried replacing the screen and cables, as well as triple-checking that everything was plugged back in correctly after service. The computer works just fine with an external monitor plugged into the HDMI port, and I can read the laptop screen with a flashlight.

Are there motherboard schematics I can look at to determine if the backlight is getting power? Where else can I look to fix this problem? I would like to fix it without purchasing a new motherboard as they are difficult to find for this model.