Chosen Solution
Display doesn’t come on when I boot up. Works with an external screen connected with thunderbolt adapter
That points to an internal display issue. Let’s give this a try, take a flashlight aim it through the lids logo are you able to see the faint outline of your desktop and its icons? If you do thats telling us the backlight has an issue. You’ll need to inspect the LVDS connection on the logic board for damage
Do you see any corrosion or damage on the logic board connector or cable? Follow this guide MacBook Pro 13" Unibody Mid 2012 Logic Board Replacement Follow the guide to Step 4, then jump down to Step 15 so you can inspect it. Let us know if you see any damage, while the logic board is visible also check it as well for any staining or corrosion.
