First the thanks and reference notes:

Almost all information for this write up was gleaned from this (http://forums.nasioc.com/forums/showthread.php?s=&threadid=338562&perpage=50&pagenumber=1) thread on NASIOC.com, and if you want to know how and why this works, read that thread. "vrg3" designed the circuit i'm demonstrating, and his initial write up can be found here (http://www.graphics.cornell.edu/~v/dimmer/). Thank him profusely for coming up with this elegent little circuit, not me.

Why this is needed:
Subaru likes to switch circuits on the ground side. Problem is that with Omori Electric gauges, the ground for the illumination circuit is shared with the ground for the gauge itself, so if you were to hook it up to the car's dimmer circuit, you'd be cutting power to the gauge as well as the lights, causing it to malfunction or not work at all. On top of that it would dim backwards, being bright when the dash is dim, even if you had access to the ground of the bulb.

The solution is to invert the voltage from the cars illumination circuit and use the output to power the positive lead of the gauge's illumination, so it receives the variable positive voltage it expects.

Disclaimer:
I'm using this circuit in my '04 WRX Wagon with two Omori 52mm Electric multicolor gauges; an EGT and Boost. Works great for me and others. It should work with any aftermarket gauge that needs positive variable voltage for its illumination circuit. Anyone with basic soldering skills should be able to pull this off.

What you need:

(4) 10k Ohm resisters
(1) TIP31 transistor
(1) LM741 op amp IC
a few feet of 22AWG wire at most
soldering iron
solder
electrical tape or shrink tubing

The circuit parts can all be purchased from Radio Shack for under $10. Here's a pic of the parts from Radio Shack:

Here is a simplified layout of the circuit you'll be building.

NOTE the pin number ordering on the LM741 IC chip, pin 1 has a small dimple next to it on top of the chip, which is marked by the notch on the upper edge. The pin layout of the TIP31 transistor should be on the back of the box... this diagram shows it with the metal heat sink facing towards you.

Now to start soldering!
First gently bend the pins of the IC chip so they are sticking out parallel. Put the LM741 face up and the TIP31 face down (the heat sink metal facing up). I used a piece of duct tape to hold the transistor to the table so things wouldn't slide around everywhere.

Solder:
pin 8 to the Emitter (E) pin
Pin 7 to the Collector (C) pin
Pin 6 to the Base (B) pin

Now bend the leads of two of the resistors so they look something like this:

Solder one resistor between pins 2 and 8, and one between pins 3 and 7, like so:

Now flip the circuit over, so the TIP21 is facing up with the heat sink down.

Solder a resistor between pins 3 and 4, like so:

Solder one lead of the fourth resistor to pin 2

Cut your wire into 4 even lenghts, I used 22 AWG wire, each lenth about 6 inches long. Strip about a quarter inch off of the end of each piece of your wire.

Solder one wire to pin 8, this is your output wire

Solder one wire to pin 7, this is your illumination (+) wire

Solder one wire to to pin 4, this is your ground
Solder one wire to the lead of the resistor coming off of pin 2

Pins 1 and 5 are NOT USED. You can clip them off if you'd like.

Here is the completed circuit from the top side of the LM741

Here is the completed circuit from the bottom of the LM741

NOW... how do you wire this thing up to your gauges?!

The ONLY WIRE that actually connects to your gauges is the output wire that is connected to pin 8.

The white (+) 12V light/small lamp wire on the Omori Harness connects to the output wire of this circuit, and to nothing else! The other wires of this circuit are explained below.

Here's the basic connection list for Omori Elect. Gauges:

Omori White --> dimmer circuit output lead
Omori Black --> ground
Omori Red --> ignition switched power
Omori Brown --> external warning lamp if desired, or leave unconnected
dimmer circuit Illum (+) --> OEM (+) (Switched lights)
dimmer circuit Illum (-) --> OEM (-) (Dimming Lights)

Now where is the OEM dimming circuit? Keep reading.

The dimmer circuit on my '04 WRX was easily accessed behind the stereo via an unused plug which i assume is for the OEM Boost gauge (correct me if i'm wrong folks).

First remove your stereo (instructions for which are everywhere, search scoobymods.com). Against the back wall is a T shaped socket plug with Orange and Violet wires. Mine was taped to another wiring loom with white tape.

The Violet wire is the cars Illumination (+) and the Orange wire is the cars Illumination (-).

Connect the Illum. (+) coming from pin 7 to the Violet wire, and the Illum. (-) coming from the resistor on pin 2 to the Orange wire.

I soldered some spade terminals i had laying around onto the end of the wires coming off the built circuit, and just taped them in. You could cut the plug off or just tap the wires as well.

The ground wire from the dimming circuit should connect to ground. I used the ground wire that goes to the stereo.

This was the layout of my Omori Harnesses before putting it all in the car. I wrapped electrical tape around the entire circuit before installing it in the car to ensure it didn't short out on anything.
(look for a write up of the full install soon)

That's it! Your done.

As a side note, my gauges were a little bit brighter than the stock gauges. This was easily fixed by turning down the brightness on the color box. But if you have the regular Omori gauges that don't have a color change box, you should be able to get the same effect by inserting a 2K ohm resistor between the dimmer circuit's output wire and the Gauge's white Illum (+) wire. I tested this on my gauges and it was just about the right value resister. The dimmer you want it the higher value resistor you should add. Let us know if you find just the right resistor value that matches perfectly.

Phew. Enjoy...