The keyboard backlight connection is via the trackpad cable on the logic board which splits with one side going to mid board to be spilt into two connections. The larger is the keyboards data cable the smaller is the backlight connection. Review this guide Step 9 [guide|142682]
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I would inspect the cable connections as well as damage to the both the Trackpad cable as well as the keyboard backlight cable on the mid board.
The keyboard backlight connection is via the trackpad cable on the logic board which splits with one side going to mid board to be spilt into two connections. The larger is the keyboards data cable the smaller is the backlight connection. Review this guide Step 9 [guide|142682]