My guess is that the screen unit is defective. I had the same problem during one of the retrofits I did. The screen was blank although it powered up okay. Turned out to be a defective LCD screen. Luckily I was able to get it replaced. Where did you get it? There are a lot of defective parts floating around on ebay...

The MKIII units are very easy to damage. Simply plug or unplug the unit and even that can kill it. The MKIV's however are more reliable. I have yet to come accross a defective one...