First off, are you talking about the stock Subaru radio? If it was mentioned somewhere in this thread, I didn't see it.
Here's a
pin-out for the factory radio harness in the Legacy. It shows the car's wire colors for the different model years.
For older Legacies, see
this pin-out by Vikash (on his new site). Here's the drawing:
![Image](http://img140.imageshack.us/img140/7528/subaruoldradioharnesspiof3.gif)
(view is looking at the pins on the car's harnesses, wires pointing away from you)
For quite a few years (even before 1993) Subarus have had two wires for dash and radio illumination:
- "Illumination +" gets +12V when the parking or headlights are on, open-circuit otherwise.
- "Illumination -" changes voltage as you adjust the dimmer knob on the dash. It varies from about 10 volts (with knob at the dimmest setting) down to 0 volts (with knob at brightest setting). Note that this is reverse from many other cars (where 0 volts = dimmest illumination).
The radio illumination lights (and instrument panel lights) are connected between these two lines. So the bulbs effectively "see" a range of 2 volts (dimmest) to 12 volts (brightest).
There is more information in this thread I posted on NASIOC:
Stock head unit information -- pin-outs, subwoofer, aux inputs, and more.
After-market radios are fairly simple: Most (but not all) have a single "dimmer" wire. When it gets +12v, the illumination dims slightly. It is not used to power the lights directly (they will work even without this wire connected). If your after-market radio has a dimmer wire, connect it to the Subaru's "Illumination +" wire. On the after-market radio's harness, this should be an Orange wire, but check your radio's manual to be sure. There is usually also a menu option on the radio to enable the dimming feature (or set to always bright or always dim).
To make the factory radio light up with the dash lights, but always be as bright as possible, simply connect the radio's "Illumination -" wire to ground.
I haven't been able to test in a Legacy, but Vikash says that the factory radio's lights are not as bright with the headlights on (with the dash dimmer knob at full bright) compared to when the headlights are off. In that case, you would need to interrupt the "Illumination +" wire during daylight, with either a simple switch or a fancier photocell circuit.
David C.
1996 SVX L AWD Laguna Blue Pearl