This is the result of a question posted on the Legacy Central forums.
I have an aftermarket boost/vacuum gauge in my 1993 Subaru Legacy Turbo. It's got a little light bulb in it for illumination, which has one wire coming off it for power. It grounds through the chassis. I hooked the wire up to the orange (illumination) wire coming off the Metra harness adapter that I used to hook up my aftermarket stereo. The light came on and off with the headlights, but didn't dim with my rheostat control.
Subaru does things weird. They have this obsession with switching electrical circuits on the ground side. It turns out that both lines going to the bulbs which illuminate the dash are controlled. One comes from the battery through the illumination relay, which is powered when the parking lights are on. That makes this line +12 volts when the lights are on, and floating when they're off. The other line comes from the illumination control module, and is modulated to be close to 0 volts (ground) when the dash lights are to be their brightest, and close to 12 volts when they should be dim.
If you wanted to install an aftermarket gauge and have it dim correctly, you'd be in good shape if the gauge gave you access to both wires going to the bulb (and neither of those wires touched ground internally). If, however, your gauge is like mine, and you hooked the variable voltage wire to its input, the gauge would dim backwards -- it would be bright when the dash was dim and vice versa. I guess this is why Metra just took the other line (the one that's +12 when the lights are on) for their orange wire.
I built a little circuit to "invert" the voltage from the variable voltage wire so I could get my gauge to dim right:
I just got the LM741 and the TIP31 transistor from Radio Shack. The chip, transistor, and resistors would run you about 4 bucks from Radio Shack, and about $1.50 from a more reasonable source. Pretty much any old op amp and NPN power transistor should do, and the resistor values aren't critical as long as they're all the same. I wouldn't put too much load on it, but a few small light bulbs should be okay, especially if you put a heat sink on the transistor.
I just assembled all the components together, and put on a connector that would plug into the harness meant for the ashtray light, since I took my ashtray out a long time ago to install a graphic equalizer:
When building the circuit, I took advantage of the fact that pin 8 of the LM741 is a "not connected" pin. I just used it as a tie point to simplify the design. This means that these are the connections made:
Built this way, the circuit can be easily sealed in epoxy.
Note that this circuit requires a load on the output in order to function correctly. You cannot simply hook it up to Illum(-), Illum(+), and ground and then measure the voltage on the output wire. If you wish to test it without installing a light bulb, place a resistor between the output and ground. 10K ought to do it. Or, you could put a much larger resistance (maybe around 500K) and leave it in place even when you connect the load.
Some other Japanese car manufacturers (Toyota, for example) use a similar system, so this circuit would probably be applicable there.
NASIOC member skivvie wrote up a beautifully detailed how-to guide on assembling this circuit that should help novices get it right. It's posted on ScoobyMods:
http://www.scoobymods.com/forums/showthread.php?threadid=3115
Note, however, that I disagree with his recommendation at the end that you insert a resistor in series with the circuit's output to reduce static intensity. That technique requires the resistor to dissipate the power to make up the difference in voltage, and requires a resistor specific to the particular gauge (or combination of gauges) being used.
The way I recommend reducing brightness is by changing the voltage going to the noninverting (+) input of the op amp. That is, either increase the resistance between pins 3 and 7, or decrease the resistance between pins 3 and 4. If you do one or the other even a 1/8-watt resistor will be okay.