Subaru doesn’t over-design their electrical circuitry, so the only way I found to run my HIDs whenever the headlight switch is on, and with high beam, but NOT with the flash-to-pass without overloading existing wiring is to use
a 30-Amp relay (Radio Shack 275-226)
an 18 gauge wire running through the firewall
and 2 – 1A diodes (Radio Shack 276-1101)

Here’s what I did. Hopefully it’s easy to follow, but it’s not meant to teach wiring. Comments are welcome.

One screw (underneath, left) is use to hold the cowling on the steering wheel column. Remove the screw and separate the cowl. You will have to “unlock” the steering wheel tilt lock to be able to remove the cover completely.

The most prominent electrical connector contains the YG (Yellow/Green) “power” wire mentioned above and GR control wires going to the multifunction switch.

Since I wired some separate DRL lights (yes, I believe in them), but didn’t want these to light with the headlights, I disconnected the YG wire. You might see that in the picture. But that's not related to this mod.

I used 18 gauge wire for the relay switch circuit. The picture showing the GR (Green/Red) wire is pretty self explanatory. But be careful, there are 2 GR wires on this connector. The relay switch ground lead is tapped from the GR wire on pin 13 (middle of connector). I really should have used a black wire here because this is for grounding. The diode in the GR wire is to keep the relay from being activated by the flash-to-pass switch. The diode in the relay switch ground wire is to keep the relay from being activated by “reverse logic”, like headlights off / parking brake on, or when key goes to “start”. This one caught me as a “duh”. Maximum current in this GR wire is 0.5A. Switch current for the relay is 0.1A. I used 1A diodes for each.

The relay switch positive lead is tapped from one of the headlight relays. It was handy under the main fuse box. One could also use an IG wire, but the one I used is protected by the 15-amp headlight fuse. Near the rear of the main fuse box is a 8-pin connector with leads from the headlight relays. I used the RL (Red/Blue) one from the left headlight relay. Since it’s under the hood and amongst other critical wiring, I soldered it instead of using a vampire tap. In the picture the RL wire is disconnected, but it lives in pin 2 of the connector, near the middle of the clip-side, next to the LR (Blue/Red) one.

I used 14 gauge wire for power to the ballasts. The power lead for the relay was taken directly from the battery with a fused link. I’m currently using a 15A fuse for the 2 Subaru STi HID ballasts. The ground leads for the ballasts is run directly to the body from each ballast.