think you might have a failed screen inverter
(FL inverter); this problem is very common with laptops. The inverter board acts as a power supply for the LCD screen backlight bulb, it converts low voltage DC power into high voltage AC. On most laptops the inverter board is located inside the display assembly below the LCD screen and you can get access to it if you remove the screen bezel (LCD mask). When the screen inverter goes bad the backlight will not turn on anymore or it turns on for a short time and then goes off, but you still can see a faint image on the screen and the external monitor works just fine. First of all I would make sure it’s not just a loose connection and reseat connectors on the inverter (it looks like you already done this part). Next, I would try replacing the inverter board. It might fix your problem with the backlight. It’s possible that you have a faulty cable, bad motherboard or even bad LCD screen, but I would try replacing the screen inverter first.