Order a new keyboard backlight, this is the individual part which lights up behind the keys. Chances are, a couple of LEDs have failed, or the backlight sheets are messed up, probably from minor liquid damage directly onto the keyboard.
Make sure you order an original new (or used) backlight, with the silver backing. A lot of places sell copy backlights (with white backing) which will have uneven lighting also.
After trying a backlight, test it to make sure it is fine before completely reassembling it. Could be gunk behind the keys, so might be worth popping a dim key off carefully and checking.