I've been having a strange issue with iPhone 6s backlights coming on and going back out seemingly at random until I noticed something. If your 6s backlight doesn't come on after a screen replacement, unplug the camera/speaker cable and retest. If it suddenly works, you're shorting out the backlight. The large chip on the back of the 6s screen cable CANNOT contact the upper cable assembly in any way. For some reason, this tends to cause a short between the camera flex and the backlight. When the camera flex cover is in position, and that small black strap pulls the cable assembly up and snug, you have to make sure it's straight across. If that long, rectangular chip contacts the assembly, it'll cause problems.
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