Aspire 5100 Display...What's failed?

The display on my Aspire 5100 only lights up sporadically. When it does light up, it works fine, but won't light up on a reboot. There is absolutely no problem hooking up to an external monitor....that's how I'm using the computer for this message.

Background: I was having problems due to dirt/lint accumulation around fan unit. I cleaned it out, and shutdown problems went away. But, I ended up restoring the system using Acer restore discs that I ordered. In the process of restoring the system and downloading Windows Vista updates, the screen would work at times, but would die on a reboot.

I took it to a shop, but they just want to replace the display....but I'm not even sure they've opened the computer to check inverter, backlight bulb or lcd. I do know that when the screen is dark that there is no evidence of any backlight at all.

Are there any connections that should be checked that could be the problem? If so, where would they be located i.e. lcd screen, under keyboard or bottom of computer?

How do I determine if it is the inverter or backlight? I'm baffled by the fact that even on the morning of the day I took it to the shop, when I booted the system, both the lcd and the external monitor were working. I disconnected the external monitor and used the laptop display for about an hour....and the screen worked perfectly. But, when I tried to restart....black screen again.

ATI software is working fine....i.e. when the lcd lights up, it recognizes the lcd as the primary display,and my external monitor as secondary. When the lcd fails to turn on, the software recognizes the external monitor as the default, and cannot detect the lcd at all.

Any suggestions and help would be greatly appreciated.

Thanks
Steve