Many moons ago, I installed a used Lamco boost gauge in my 2007 WRX. I plugged the electrical harness that came with the gauge into the dongle in the back of the head unit, and it didn't work. Assuming maybe an early boost gauge with a burned out light bulb, or an inappropriate use of that dongle, I left it alone, and tolerated not seeing the gauge at night.
A few weeks ago, the backlight turned on and started working.
But not consistently. Most of the time, the light still does not turn on when I turn the headlights on. However, if the headlights are on, the boost gauge will illuminate after I give an input on the left stalk (the one that controls headlights and turn signals). I've had the backlight turn on when I flashed my brights, and I had the backlight turn on when I used a turn signal. I don't get it.
Anyone have insights as to the source of this gremlin? When the boost gauge backlight turns on, it will work properly until I turn off the ignition. I can turn the headlights off and on, and the boost gauge will work correctly, turning off and on appropriately. Similarly, when the backlight is on, the dimming works perfectly.