I know I've seen a few other installs with the JDM tank or the RA tank that mounts behind the seats, but I figured I would document mine. I used the USDM 04-07 tank.
-Gates 27042 (5/32 * 50' Windshield Washer and Vacuum Hose)
-OEM Check Valve (86634AG010)
-Hose fittings (T fitting, Straigt Fitting)
-4 pin relay w/ pigtail
-5 pin relay w/ pigtail
-Boost Pressure Switch (Zex-82081 is adjusable from 2-25psi
-04-07 STI Intercooler Timer w/ pigtail (88017-FE040)
-Manual Spray Switch for GC (83001fa141)
-Auto Spray Switch for GC (83001FA111)
-04-07 STI Spray tank
-Intercooler Shroud with Nozzle cutouts (Iwire if you have an STI TMIC)
-Wire (I used 14 gauge for power to pump, and 16 gauge for everything else)
You'll need a few other supplies too like electrical tape, heat shrink tube, solder etc... Also if you have a GD dash you will want buttons for a GD dash. Those will be a lot easier to find than the GC buttons (which are no longer available through Subaru). I also welded metal tabs to my trunk to install the tank. You could probably find another way to secure it if you didn't want to do that. I have a piece of plywood covering the spare tire area that I cut using the oem fibeboard as a template. I thought about cutting a new piece of plywood and extending it over on the passenger side so that I could just mount the tank to that instead.
This started with the JDM diagram and then I tweaked it a little. The JDM ecu MAP, ECT, IAT, and RPM from the ECU to control auto mode. The stock US ECU won't have this ability so I used a 2-25psi adjustable Hobbs pressure switch for mine. I also eliminated some of the junctions and connectors not needed in the diagram and added my own text to explain how it works.
I made a video testing it out on the workbench too:
Sti intercooler sprayer wiring for 2.5rs (Manual and Auto buttons) - YouTube
Here are the brackets I had welded in. It's just 1/8" steel stock that I cut and drilled/tapped.
This is the carpet I had to cut for the mounts.
Here is the tank mounted. I also sprayed the primered mounts black.
This is where I mounted the 2 relays and Timer. The timer is mounted to the other side of the aluminum extension arm I made. You can see I have the switches all wired in too. Get everything mounted before running any of the wiring. That way you can cut it to length.
This shows the button height difference. I mistakenly bought GD buttons thinking they were all the same. The GC buttons are shorter. I was able to track down some used buttons on ebay (still waiting for them to arrive). You can no longer buy them new. I confirmed this with japanparts and rallispec. You can see the hobbs switch in this picture too. You use 2 of the pins for it. When it sees X amount of boost it will trigger continuity between those pins. It can be configured between 2-25PSI be turning a tiny allan bolt. I set it to 17psi using my air compressor, and an old gauge I had laying around, and a steady hand.
This is the gates hose. I ran it through the firewall where an existing rubber plug was behind the intercooler area. The brake lines run up the drivers side of the car and there are some grey clips that hold them in. Fortunately there is an extra spot right beside the brake lines. I had to bend/stretch the opening a little wider on the grey clips so that the hose would fit and it wouldn't be pinched as tight.
Here is an underhood picture. This is the iwire shroud meant for a gc hood and sti top mount. I bought the spray nozzles from them as well. They will also sell pigtails too if you need them. They are about $10-15 each though so keep that in mind when looking at switches. It would be worth paying $5 more for a switch that comes with a pigtail. If you don't have cruise control you can re-purpose that pigtail for one of the switches.
edit: switches came