Back Light and Screen Issues

The problem went through two stages.

1. Screen back light didn’t come on at all (completely black) but I could see what was on the screen my using my phone light. Could log into mac os etc.
2. Then the problem seemed to shift - either the laptop came on and screen remained black (phone light through the back didn’t work), or the screen did come on, but only for periods of about 1 minute or less.

I seem to be stuck in the second stage now. The phone light isn’t working.

It doesn’t work with an external monitor when the screen is black.

I believe the screen itself is actually ok. I got that replaced 2 years ago. This seemed to happen all of a sudden. It went from working to not working without any warning signs.

Just wondering if anyone had any idea what to look for or try. Opened the back up and nothing obvious.

Thanks in advance to anyone who replies!

MacBook Pro 15" Retina Display Mid 2012