iMac built in display completely black unless booting Mac OS

Hey everyone,

I have an iMac 10,1 (Late 2009, 21.5 Inch, Core 2 Duo E7600, ATI Radeon HD 4670).

The problem is that I've been trying to get Windows on it but the internal screen ONLY works with a UEFI installation. Otherwise as soon as I boot either into a Windows legacy setup or legacy installed disk my internal screen goes completely black, no backlight whatsoever.

It's like it's disabled, not even recognized in settings. Windows only detects my external monitor. I've confirmed this isn't only a Windows problem but rather booting a BIOS (Legacy) device in general. For example - Plop Boot Manager disc or Zorin Linux install disc. Both turn off my internal screen and show only on external monitor.

If i try to install my GPU drivers on the UEFI installation, then both screens are detected but the internal monitor display is severely corrupted (Image).

At first I believed it was a Windows driver problem. However I now suspect it's a EFI issue.

Would someone please be kind enough to offer any input/advice to the problem I'm having?