Yes, it's entirely possible the Left I/O Board (official name) is the problem; a while ago now, I had a fellow technician find out on a similar MBP that the Left I/O Board was preventing the back light from functioning. Replaced the board, and viola. Of course, it's not a guaranteed diagnosis. You'll have to change the part to see if it's the problem. Has the inverter board been changed? That part is also often the problem when it comes to back lighting issues.