Found this writeup which may be relevant:
Install Instructions
The TX is flashed with the latest er9x firmware.Starting with Rev 2.2, the backlight is designed to operate on by default to make it compatible with all firmware options. However, in doing this it makes some firmware versions work "backwards". Until a given firmware package has specific support for the Rev 2.2 board, you may have to program it to be "off" when you expect the light to be on, and vice versa. Some technical jargon to help firmware authors: The light is powered when the 9x outputs a low (zero volt) signal on PB7 (aka "pin 17") . A high (5v) signal turns the light off. Sending a PWM of varying duty cycle will dim the light from 0 to 100% (frequency of 400hz if frequency can be set independent of duty cycle).
Is there a way to toggle PB7 / pin 17 via one of the hardware switches programmatically?