Black screen after screen replacement for MacBook Pro Retina

The screen on my MacBook Pro 15" Retina Display laptop (Mid 2012) was damaged so I had to replace it.

The MagSafe magnetic charger head was out of the charging port and on top of the keyboard when I closed the lid and just a little scratch on the lower right corner killed the entire screen. I could see just a gray screen.

I purchased a replacement screen from eBay (not just the LCD, the entire screen). When it arrived, the LVDS connector cable was damaged. I should have returned it right away, I know, but the genious technician at the shop where I took my Mac decided to give it a try anyways. We uninstalled the existing damaged screen and installed the new one. When we turned on the laptop, you could hear the start up chime sound but no screen, just a black dark screen, no Apple logo or anything at all.

Then we took the undamaged LVDS cable from the existing damaged screen and installed that on the new screen, but still just a dark blank screen.

Then we also tried the existing damaged screen, which remember did show a screen before, even though it was damaged and just gray, and still a dark blank screen and nothing else.

Tried increasing the brightness from the keyboard buttons.

Also tried a few things from online recommendations like resetting the PRAM reset or other things mentioned in this article:

http://osxdaily.com/2014/11/22/fix-macbo...

Connecting to an external monitor works and the display looks good and the machine fully operational.

At this point I suspect that the new replacement screen with the damaged LVDS connector cable might have damaged something else on the laptop's motherboard, but I have no idea what it could be and where to start.

Any ideas or suggestions on what it could be?