No Video on LCD

Hello, I have a A1224/EMC 2133 Core2 Duo 20" iMac that will not produce video on the LCD. I bought this system with a known no video issue with the intent to fix it. I am a computer repair tech with over 15 years of experience, just looking for some second opinions on how to proceed. When I originally purchased the system it would not produce video at all on LCD or external monitor, but the chime could be heard. I replaced the video card with a used x2600 card from eBay. the original card was a 2400XT. Now it outputs external video but no video on LCD. Below are the troubleshooting steps I have performed so far.

-System will produce video to an external monitor and boot to a setup disk

-Resetting PRAM had no effect

-There is no video on LCD at all. Using a flashlight, there is no ghosted image to indicate there is simply a backlight failure

-The backlight does not appear to be turning on

- Diagnostics LEDs 1,2 and 3 light up right away and stay lit up. LED 4 never comes on.

- Voltages on the power harness going into the LCD inverter are Pin1:3.3v, Pin2:0v, Pin3:-.03v, Pin4:12v. I may have the pins listed in the reverse order.