This gauge was installed in a MY00, and with great assistance from Vinnie Paul whose website helped also.(http://members.optusnet.com.au/~vinniepaul/)
In addition to the gauge, I also used the following:
* decent wire
* good wire connectors
* 5 metres of vacuum tube (obtained from Autobarn)
* PTFE tape
* cable ties
The gauge will come with the right brass fittings.
First off all, your gauge should come with some hose to connect it to, you need to route this to the engine compartment. Where you route the hose depends on where you want the gauge, I installed a gauge in a pillar mount. You need to take off the bottom half of the dash. Open the door and pull off the plastic rivet type thing holding the bottom of the dash in. It looks like it screws out, but it doesn't. In fact, it proved to be a real pain in the arse. Get a flat head screw driver under it and slowly lever it out while screwing the top of it with a Philips head.
There are another two screws, under the dash. There is one in front of the bonnet catch, and another on the other side. Pull the whole section out, it has plastic clips so it feels like it is still connected, it isn't. Disconnect the plugs so it can sit on the floor. I didn't disconnect all of them, I just moved the section around as I needed to.
Get your hose and stick it through the top of the dash near the pillar and the window. There is a gap big enough if you get the right spot. The hose that comes with the gauge (plastic) is better for this as it is much more rigid. I also found it better to do from dash down, rather than upwards. Just keep shoving it in there, it can be tricky to find from under the dash.
I then joined this plastic to the rubber hose and sealed it with a cable tie - no problem with any leakage. Running the rubber cable along the entire firewall up to to the aircon entry point, I pushed it through. VP recommends using a skewer - probably a good idea, but I used brute bloody force.
Then the hose is run following any line up to the engine manifold. Photos shows the place - I was looking all over until I got hold of a photo - then it was obvious.
In the engine bay, yank off the little cap over the prong on the manifold. If you have a earlier model WRX without this you will need a T-piece and use another vacuum hose. Likewise, those WRXs that have a vacuum driven cruise control will also need a T-piece to tap into this line. Then stick the vacuum hose onto this, then try and route it sensibly towards the grommet. Tie it in with cable ties. Connect it to the hose from the dash.
Now you need to connect the light of the gauge to the dimmer. First of all stick 2 runs of wire down the dash next to the vac hose, and get under there and bring them around to roughly in front of the fuse panel. I used the connections from the rear foglight switch under the dash(yellow), or you can use the front foggies(black). Alright looking at the connector with the four wires at the top, the dimmer power is the 2nd wire from the left (purple), and the ground is the bottom right hand side wire (yellow I believe). Cut these, strip and join with connectors to the wires going to the top. As its only a bulb it doesn't matter what wire to which. Use heat shrink or electrical tape to insulate. One point to note. It DOES matter if you want the gauge lights to dim with the instrument lights. If you find it looks funny, or does the opposite, then reverse the wires. The WRX uses a system that alters the earthing of the wiring to achieve various brightness levels - if it all looks bad, swap the wires around.
I would not recommend silicon. I used double sided tape/pads on the A-pillar. I also got some adhesive foam (insualting foam for doors etc is fine) and cutting it to size, stuck it around the bottom of the pod. This not only stops any potential sqeaking, but holds it firm as well. This and the double sided pads work well.
You should get about -15psi approx reading at idle (this will vary though). I find that average when warm is -15/16psi, but it can drop to -20psi after a bit of a blast. Max boost (unmodified) of around 13.5 psi.
That's it. All good fun.