Some 2.0 GHz A1278 MacBook Unibody logic boards do not have the necessary socket installed to accept the keyboard backlight ribbon cable attached to the upper case. The first and second pictures show logic boards with and without the socket installed, respectively. If your logic board does not have the backlight ribbon cable socket installed, the keyboard will work but the keys will not light up.

If your logic board does not have the socket installed to accept the keyboard backlight ribbon cable and you are installing a backlit upper case, simply tuck the keyboard backlight ribbon cable out of the way when you reinstall the logic board into the upper case.