BacK Light Issue

[Doodster]

I have replaced a lcd screen on my daughters Sony vaio laptop. The screen is black but when you shine a flash light on it you can see the computer is working. At first I thought it was the screen so we got another screen fpr a simple install, turned the computer on and its still black.. So I hooked it up to an external monitor and it works fine on the monitor.. So I began to look in forums and from what I am reading there is a lid switch that could be faulty or stuck.. If you could point me in the right direction on where to look for the switch and or fix for this thing Me and my Daughter would appreciate it..
Merry Christmas
Doodster

Sony Vaio
Model# PCG 61313L