While we don't have a discreet guide to replace the backlight assembly the needed steps to replace the keyboard is the same. But before you dig into your system are you sure you need to replace it? I would make sure the power is present on the logic board.
I find liquid damage also takes out the keyboard besides the backlight, it just takes a bit longer.
You'll need to follow this guide to get to the keyboard MacBook Pro 15" Retina Display Mid 2015 Upper Case Assembly Replacement
Now it's just a question on just replacing the backlight or also replacing the keyboard as well. The backlight is glued to the uppercase so once you get things out of the way you can carefully lift it off. If you only have one section out you might only need to replace the tiny LED and put it back. Apple never offered the back light as a replaceable part so all you'll find is used ones which is getting hard for this series as even today people are holding onto this great system! I have two and I'm not likely to part with either for a few more years!
If you think you also need to replace the keyboard its a bit a chore! You'll find a massive collection of screws and plastic rivets holding the keyboard assembly into the upper case. You'll need to replace the rivets with screws as well as removing any stuck within the holes. It takes a fair amount of labor and patience MacBook Pro 15" Retina (Mid 2012-Mid 2015) Keyboard. The other options is just replace the uppercase unit which I find is much faster but more costly MacBook Pro 15" Retina (Mid 2015) Upper Case Assembly
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