Half of keyboard backlight dim after soda mishap
Unfortunately, there's not a lot that you can do. Liquid damage is tough to repair without replacing components. The only way to get to the underside of the top case is to remove every component in the machine, which I'm guessing you're not set up or trained to do. Even if you could get to it, it's unlikely that you'd be able to restore the backlight. The best thing to do is to take your machine to the Apple Store and tell them exactly what happened. Since liquid damage is not covered by AppleCare, you will have to pay for this. Sometimes they are able to help you out with the repair cost, but not always. The worst case scenario would be sending the machine to an Apple repair center for a complete repair of all components affected. This costs a flat rate of around $1250.

I know this isn't the news you were hoping for, but I hope it helps!