I've read reports people changed battery and the auto-brightness stopped showing. Try to investigate on thatI've recently changed my iPhone 5 battery( in May, noticed that it's missing recently after resetting all settings), I've reset all settings, reinstall iOS via iTunes too
I don't have any tools to open it, I've just DFU restore my iPhone 5 and set it up as new,no auto brightness toggle.I remember having exactly this problem with my 5 when I attempted a repair - I had to replace a dead microphone near the camera, and changed the battery at the same time.
I cured it eventually but annoyingly I'm struggling to remember how. I think it have been as simple as disconnecting the battery again for a minute or two and then reconnecting.