Bad LCD backlight or motherboard?

I dropped my Nikon D3100 and broke the lens, cracked the outer LCD glass and killed a few pixels of the LCD screen. I've been trying to slowly repair it (mostly because of the sentimental value of the camera).

I ordered a replacement outside glass from China but when I replaced it the backlight on the existing LCD screen stopped working. The screen itself seemed to work OK (by shining a light into it), but the backlight was dead.

Thinking that I probably had to replace the screen given the dead pixels I ordered a replacement screen with backlight, but once again the backlight's not working. I've used a multimeter and the voltage going into the LED array is 9V, so there doesn't appear to be anything broken in the circuit, but no idea if this is enough voltage. By shining a light on the screen I've managed to reset the camera and increase the screen brightness but to no avail.

I've seen comments that sometimes these replacement screens come with faulty backlights, but I’m not sure if the fault is with the backlight or maybe the motherboard.