A common issue with Apple laptops is no backlight due to enable resistors being blown going to the LED driver. This only happens in the case of liquid damage. We repair these issues under our Macbook logic board component level repair service.
The LED driver requires 3v on the enable pin in order to activate. This circuit runs on 12v. There is a transistor that sends 12v to the circuit when the system wants the backlight to be activated. There is no need to recreate this circuit using transistors from a 3v rail, this would add to the cost. It would be cheaper to use a simple voltage divider comprised of two cheap resistors to send the signal to the LED driver when the 12v to the backlight circuit is switched on. This is what R9731 & R9715 do on many Apple boards.
The damage occurs on the 12v side of course, and will usually destroy the pad on one side, which may have to be scraped away to make the circuit work.