Chosen Solution
Recently, I replaced the upper case following ifixit’s guide and using a case bought from ifixit’s part store. The case was a little bent near the magsafe port, but i was able to bend it back into shape easily enough. Everything seemed to go fine. The keyboard backlight lit up consistently for a number of days. (I typically leave it off unless I’m in a dark environment, so I don’t have an exact amount of time) Now nothing. When pressing the function keys that correspond with the keyboard brightness level adjustments the system pops up the brightness slider and adjusts up and down accordingly, but the keyboard backlight itself doesn’t respond at all. I also recently had a message pop up saying the system was shutting down a USB port because an accessory was pulling too much power when nothing was plugged in at all. I’m not sure if the two are related somehow. I have already reset the SMC and PRAM. I also opened it up to check for loose connections, but found nothing. Anyone have any ideas as to what’s going on?
The problem you are facing sounds like you have a power rail issue which supply the power for the USB ports. Did you use any USB devices which have their own AC power bricks? You may have used the incorrect brick for that device causing the power to enter into your system via the USB port causing damaging the USB power service. Lets see if the onboard diagnostics tells us anything. Restart your system and press the D key to enter into diagnostics. Tell us what you get.
