Starting in 2008 (same year Subaru changed to the new 20-pin harness for the radio's power and speakers), Subaru switched to LEDs for the instrument illumination. LEDs do not dim well by simply varying the voltage. So the car's dimmer circuit was changed to a pulse-width modulated (PWM) signal -- +12V switched rapidly "on" and "off". This is the proper way to dim LEDs. The car's dimmer wheel adjusts the percentage of the LEDs' "on" time (the "duty cycle"). It happens so fast that the LEDs simply appear dimmer or brighter.
As in older years, the car's illumination(+) signal is simply +12V when you turn your headlights switch to either the parking lights or headlights position.
The car's illumination(-) lead is the PWM signal (a square wave, alternating 0 and +12V). Frequency is 250 Hz. Note that since this is the "negative" (low) side for the LEDs, brightest illumination happens when the PWM signal spends most of its time at 0 Volts. For dimmest lights, the PWM wave spends most of its time at +12V.
So if you can separate both sides of your gauge's LED, you can connect it across the car's illumination(+) and (-) leads and have it dim in tandem with your other instruments. You may still need a series resistor to limit the current through the LED.
If you can't or don't want to split off the gauge LED's ground lead, you might still be able to connect the LED's positive lead to the car's illumination(-) signal. You would need to invert the car's PWM signal (so that you end up with a signal with higher % time at +12V means "brighter"). It would be easy to do that with a simple transistor circuit. As a bonus, you can use the illumination(+) signal (or even +12V Ignition) for your circuit's +12V supply to the LED(s), and then you won't have to worry about possibly drawing too much power from the car's illumination(-) circuit.
Check with your gauge's manufacturer to make sure they don't already have some funky internal dimming circuitry that won't like being fed a PWM signal. If they just have a straight LED (or even incandescent bulb), there shouldn't be any problem.