As far as I heard it's coused by not disconnecting the battery, therefore the backlight IC is damaged and need to be replaced. I have never tried to replace this IC nor I know where it is located on the board.
first of all try to install a new screen to see if it helps. there is no harm in trying a new screen. if it didn't help any, try re-soldering the chip back in the place, maybe you missed something while soldering the IC. if yet it's not solved then I think it safe enough to say that It is indeed the touch IC.