Why does my screen flash on and then go dim?

Hi i have been having a issue with my i mac for a while now, basically one day i switched it on and the screen would light up for a few seconds and then go dim ( and leave a faint image in the background if i use a torch ) so i came straight onto Google and researched it. Many people said do the 2 resets--> apple button alt, p, r, so i tried them no luck so looked into it more and seen was likely the inverter so ordered correct replacement followed the instructions to replace it and it does the same thing....

I will add a you tube link to the a video i made of it just to clarify i am attempting the correct fix to the issue

http://www.youtube.com/watch?v=NVgVaNkh7...

If any help is available please feel free to comment

( I have checked the 4 green lights and they r fine )

( I have been using the computer on a external monitor and it still works fine )

( yes the brightness is all the way up )

I purchased this as replacement

Thanks again