Why is the LCD backlight not working?

I recently switched out the LCD of my iMac (21.5" EMC 2544, late 2012) with a new one. The old one was cracked but the backlighting still worked. With the new LCD in place, the backlight no longer works. I can tell that the LCD itself is fine since it has an image when I shine a flashlight on it and there's not problem with the GPU as I currently have it connected to an external monitor which works fine.

It is not a problem with the new LCD because I have tried plugging back in the old cracked LCD and there is no backlight anymore.

There are two cables connecting the screen to the motherboard and I am wondering which is responsible for the backlight and if one of these cables is not the problem, maybe there is a problem with the connection to the motherboard?

Any help would be much appreciated!