Don, first I must admit that I am not a Mac person. So this answer is a general answer and hopefully the Mac Guru's can help you out further. Since you already replaced the LCD with a working one, it is possible that this is caused by a bad inverter. Try to connect an external display and see if that works. If so, then it is the inverter/backlight circuit of your computer. If it does not, then you are having GPU trouble on your computer. Apple suggest:" No display or dim display, but computer appears to operate correctly and has video operating on external display (display is fine after startup, but after a few seconds no display or dim display)
1. Remove any connected peripherals.
2. Try known-good power outlet, power adapter, and power cord.
3. Press F2 (with the fn key pressed and not pressed) to increase the screen brightness setting.
4. Reboot the computer—hold down the Control and Command keys and press the Power button, or press and hold the Power button for 5 to 10 seconds to shut down the computer, then press the Power button to restart.
5. Reset the SMC. See Knowledge Base article 303319, “Resetting MacBook and MacBook Pro System Management Controller (SMC)”. Resetting the SMC means you will also need to reset the date and time (using the Date & Time pane of System Preferences).
Warning: Make sure you do not hold down the “fn” key when resetting the SMC.
6. Reset PRAM. Restart the computer, then hold down the Option-Command-P-R keys until you hear the startup sound at least one additional time after the initial startup sound.
7. Connect an external display, and check for video on external display.
• If video is ﬁne on external display, troubleshoot LCD panel and verify cable connections to inverter and LCD. Verify battery connector with sleep switch connection.
• If video symptom is same on external display, replace logic board.
8. Verify that LCD panel cable and LVDS cable connections are seated properly and that the cables are not damaged. Check cables around the hinges for damage.
9. Replace Inverter board.
10. Replace battery connector with sleep switch.
11. Replace LCD panel.
12. Replace logic board.
If the numbers are right, then you have a fuse for your backlight on your logic board. It is a 2AMP-32V SMD fuse. Test it with a multimeter and see if you have continuity. If not then you need to replace that. Hope this helps, good luck.