[sperry]

The dimmer for the older cars was a simple potentiometer on the grounding circuit for the dash lights. My guess is that it's the same for the newer cars, and that variable ground is simply an input to the Defi-made instrument gauges, meaning there are no individual inputs that you can use in the manner you described.

The tech page has the wiring diagrams for the '04 STI cluster, however I know for a fact there were changes made to the pins between '04 and '05. So you can at least look at the '04 to see how that works, and perhaps some minor investigation w/ a meter will tell you if the '05 is similar with regards to the dimmer circuit.

However, what I'd suggest is replicating the way the SVX does it. The SVX has a variable ground dimmer same as the Impreza that is activated when the lights are switched on. However, there is an override button on the turn-signal stock that allows the driver to manually cancel the dimmer *for everything*. That means I can turn on my lights in the day, then hit the cancel button and see my stereo, clock, A/C temp, gear indicator, etc.

If you were to install a switch in line with the variable resistor in the turn signal stock, you could flip the switch to manually ground the circuit around the dial, thereby cancelling the dimmer entirely and for not just the stereo, but for the clock and gauge cluster, etc. IMO, that's the best solution because it means the lights are on outside but the interior lighting is still in day-time mode.

Let me know if I'm not making sense and I'll draw up a diagram.