first, check and make sure you've installed all the capacitors the correct direction. i have a hunch you've maybe flipped one and put it in backwards...
also (i did this yesterday, fresh on my mind), make sure the ribbon cable is not only installed, but installed straight. they have a tendency to slip a little bit when you install them, and if you've crossed pins there you'll get wacky results on the screen most likely.
you're right, can't really mess up with installing the backlight, which would only mean the LEDs in it won't work. putting the backlight in backwards wouldn't cause the other issues.
my bet is a backwards cap. or maybe more than one.