Someone said the backlight was supposed to work with in the BIOS settings. I can't imagine there is any way that connection/cable is being damaged during the repair process - it certainly doesn't look like it. My only guess is that there must be some version of some driver that was originally installed from the factory, but it is not on the recovery partition or in the list of drivers on the website. I have painstakingly installed every single driver from the support downloads page, and tried every version I could find of the 'System Control Interface V2' and Intel chipset drivers. You have to take the battery out to check it, but that is easy to do.

There is a 4-pin connector labeled 'KB BL' just above the battery.

I took the back off and confirmed that the backlight ribbon cable is well-connected. My best guess is that when I reset Windows before sending it in for RMA is when it stopped working. Can anyone confirm whether I should see an on-screen popup when trying to adjust the keyboard backlight? I do not want to send this in for repair again and be without the laptop for another month. It seems like this has to be a software issue, but I've tried ALL possible drivers, and this definitely worked before the RMA. There is no on-screen popup when I press (Fn+)F7 to adjust the backlight (I'm pretty sure there used to be). All other hotkeys work, and the F7 key works in a test program.

I took the case apart to confirm that the keyboard backlight ribbon cable is connected to the motherboard (it was). I'm on the latest BIOS, I've reset BIOS defaults, reset Windows, tried different version of System Control Interface V2 and ScreenPad drivers. After sending my laptop in for RMA service (which took a month) because the battery died after six months, the keyboard backlight does not work.

Dell keyboard backlight not working