Hi! Sorry I'm late, this is my first post on this forum
Try this: install kernel 3.9.3 and then shut down your laptop. From the bios settings set the keyboard light mode in ON or TIMER as you prefer. Than boot with 3.9.3 and you should have your kbrd backlight back again. After this you can shut down the laptop and boot it again with the kernel you prefer, it should mantain the service active. EVERYTIME you unplug the current (by removing the battery and the wire) you'll have to boot with 3.9.3 again in order to restore the backlight mode, but always just for the first time. I hope it could help someone!
*tested on mint 17 cinnamon 64bit toshiba U940-103*