Macbook always at lowest brightness
I agree with IconDaemon's comment, so don't think there's much help you're going to find here.

However, one thing you could try is resetting your System Management Controller (SMC). Now, before doing this I should add that while resetting the SMC is usually a totally safe thing to do, you may get unwanted results in the case of liquid damage. So, if you have the option of using your MBP with an external display and keyboard, you may want to just opt for that.

Another thing you should do is ensure you have a good backup of your data. Not just because you should have a backup anyway, but also because of the liquid damage. Your MBP is at a much higher risk now of just one day not working anymore.

If you'd like to try the SMC reset, follow these steps for doing so on your model of MBP:


Shut down your Mac
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the built-in keyboard) and the power button
Let go
Turn your computer back on with the power button.


I'm assuming, of course, that the left shiftoption and control keys still work.