Marc,
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I don't remember for sure, but I think that what solved the problem for me was replacing the digitizer with a new one from a seller that I trust. I am not sure why my digitizer worked beautifully at first and then failed. Possibilities include small rips, kinks, or slightly bad/dirty connection points at the end of the cable. Or slightly incomplete insertion into a connector. This last is worth checking: open the latches on both connectors, carefully push the cable in with tweezers as far as it can go, and close the latches.
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In my case, possibly there was damage to the cable that was only revealed when the assembly was tightly closed.
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I think that I did insulate the edges of the digitizer with Kapton tape, as has been suggested elsewhere, and that did not help in my case.
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Another possibility if it is not the digitizer, is that one of the connectors is faulty. If you have a microscope or even magnifying glass, inspect the solder joints between each pin and pad, making sure that they look good and have the classic "ski slope" form of a good joint.
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If you have a multimeter, you can also test continuity between each pad and the corresponding pin. Look not only for continuity, but also the absence of any bridges: each pad should be continuous with its pin and its pin only.
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If you have ZXW and know which pins should be ground, you can test that as well. If you are fortunate enough to have a chart of what the diode readings should be for each pin (or a "known good" with which to compare) you can also test the diode reading of each pin.
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To my knowledge and in my experience, dead spots are unlikely to be caused by damage to the backlight circuit. Backlight damage is usually reflected by either a half dim screen or one that is completely blank.
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In sum, I think you are left with the digitizer itself (damage to the cable, incomplete insertion into a connector, or lack of insulation around the edges) or with one or both of the digitizer connectors on the motherboard.
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Good luck!