I head back to dell and ask them what to replace next. Sadly I think that's the motherboard.
Did you try the old remove the AC, the battery, hold the power button for 60 Seconds reset?
I am using a Dell Vostro 1510. On boot up, the laptop screen is completely blank but the display goes to the externally connected monitor. Once windows login screen comes up, the display switches to laptop screen and it works perfectly for as long as the laptop is on.
I ran the diagnostic test and I got the message:
Error Code: 2000-0321
Message: LCD EDID - Unable to access EDID EEPROM
Other posts on the net relate to the laptop screen not working at all - but, in my case the laptop screen works just fine post windows login.