Diagnostic led #4 off. No image. Screen black.

Hello. I'm at wits end.

The display started having color and line issues and occasional diminished image. So..

1. I replaced the video card and heatsink. Boots up. Now a black screen, no image at all.

2. Logic Board and Display/Screen replaced. Boots up. Screen still black, no image.

Diagnostic 1-3 green. 4th is out.

Now that I've replaced the major components involved with the problem, I don't know where to turn next. Yes, the parts I replaced were used from Ebay, but were guaranteed as working.

I read somewhere that the RAM maybe the problem, so I swapped and tested same. No change in the problem.

Any help is appreciated.