I had the same problems. Backlight was gone or very low.
The short story: I found out that the micro controller does not send the PWM signal to the LED driver.
Very mysterious! I checked the LED driver components (Mosfett, diode), I read the data sheet to understand the function of the HV9982 (LED driver). In the end there was no hardware problem there. If I connected the computer the micro controller enabled the LED driver but "forget" so send the PWM signal, so the LEDs stayed off.
Mmmm...maybe this is a software error or Apple wants to tell me to buy a new one? So I bridged the PWM input pins with a little wire connected to a 3.3 V pad (which I found about 1 cm around). Nice that die PWM output pins of the micro controller were bridged by 0 ohm resistors, so I could remove same and cut the connection from the micro controller. And then I got full backlight again. Of course I am not able to dim the backlight anymore.