Tried it with the old display? A bad LCD panel from factory can short PP3V3_S0 or PP5V_S0 to ground, which would prevent the MacBook from booting.
Did you press the button next to the battery before disconnecting the trackpad cable, so the system detect light on the logic board goes off? Not doing this has a chance of killing the board. Is this light coming on with the new display and is it getting in an S4 state (trackpad clicking)?
Check to make sure you haven't knocked anything off the board if you have tried the old display and the above is fine.
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