2009 Imac 21.5" no picture on LCD

Hi,

I now have a new issue that I have been trying to solve. First it was a bad video card, replaced that which was a task to pull the whole imac apart. But I did it successfully. A week later, I was updating the OS in software update. During the install, the computer either went to sleep, or the video went. All I know is that the LCD does not come on. The LCD shows a faint light in the dark. I managed to attach an external monitor, found a keyboard combination to switch screens. The computer runs fine, as does the new video card I put in. I reset NVRAM, SMC reset. I ran Apple Hardware Test and it showed no problems.

Is it the LED backlight driver board? I am going to double-check the pins on the conncetor cable.