Problems with backlit keyboard on E4200

I am having a problem with my old Dell E4200. I recently bought a used backlit keyboard for it (the original wasn't backlit), but the result isn't as desired. When I turn the backlight on, the Dell application lets me choose between backlight "off" and backlight as determined by the light sensor. There's also an option for backlight regardless of the light sensor, but I can't choose it. When I pick backlight + light sensor option, the keyboard goes on for maybe 15 seconds and then goes off again, no matter how many keys I am pressing.

Does anyone have an idea what kind of problem I'm facing here? Does this point to a hardware issue (since the keyboard is used, it might be faulty) or am I wrestling with the hardships of old Dell software?

Thanks!