Black Screen iMac, external working changed backlight board

Hello,

I'm having an oddly specific problem with my iMac.

I have a couple fixer uppers that I am working on, and I put a new GPU in one and put everything together only to find that the LCD wasn't working, though everything was working through external monitor and all four LEDs on motherboard were lit.

I pulled the LCD from my other computer, which I confirmed was working before switching. The LCD was still completely out, so I scavenged the Inverter board as well, which yielded nothing.

To be clear, the screen is completely dead, and I wasn't able to see my desktop at all when I shone a light on it, so I don't believe its simply a backlight problem.

Any help would be greatly appreciated as I'm not sure where to turn at this point!