Use Figure 1 to help you decide where to connect the boost gauge's vacuum pipe. There are two usual locations: one in the actuating pipe going to the recirculation valve, the other in the plenum to pressure regulator pipe. The actuating pipe is the easiest to connect, but the pressure regulator pipe is less likely to be disturbed in the future. My gauge is connected to the pressure regulator pipe, and my turbo timer to the actuating pipe. Photos of both are below.
Plenum to pressure regulator
With your boost gauge you should have been supplied a T connector and small pipe to join the gauge tube into an engine pressure pipe. You need to locate a small pipe running between the silver plenum and the engine block. Looking at Figure 2 you will see that it connects to the plenum (1) and a small solenoid (2) under a bronze valve. Remove this pipe and cut it in half. Connect the two halves to the T so they are at right angles and add the extra joining pipe to the remaining join. At this stage it is a good idea to join the gauge tube as well. Replace the engine pipe to its original location. You should now have something looking like Figure 2.
Recirculation valve actuating pipe
Figure 3 shows the alternative connection location in the recirculation valve actuating pipe. Once connected, the process is the same here on.
The vacuum pipe needs threading carefully under the plenum, as shown in Figure 4. Ensure that the tube is not in contact with the engine block. Although it should withstand the heat from the plenum, I wouldn't guarantee it for the block!
If you connected to the actuating pipe, carefully thread the pipe to the bulkhead so that it is nicely tucked out of the way.
On the bulkhead, next to the plenum, you will find a rubber grommet as shown by the red arrow in Figure 5. Remove the grommet, cut a small niche in it and thread the vacuum pipe through it. Pass the pipe through the bulkhead hole where the grommet was. From under the dash, locate the pipe and pull it through carefully. You can now replace the grommet, ensuring you have a similar amount of slack as shown in Figure 5.
Carefully remove the A-Pillar trim which is held on by clips. Be aware of the alarm sensor at the top of the pillar. Once the trim has been removed, thread the vacuum pipe under the dash and up through the narrow gap at the base of the pillar. Ensure the pipe is not restricting any controls!
Right. You've now got all your fittings to do. Three holes will need drilling, two in the GReddy pod ends to screw it to the A-Pillar trim, and one larger hole in the trim to pass the vacuum pipe and illumination cable through. I would strongly recommend screwing the pod lightly to the trim now as it will make the final fitting in the car much easier. Try to get the pod as close to the speaker mesh as possible.
Mount the gauge in the pod as per the instructions. Connect a longer cable to the gauge illumination wires and thread the cable through the pillar trim as shown in Figure 8.
Back to the car. Thread the illumination cable through the same gap at the base of the A-Pillar. Now thread the vacuum pipe through the trim hole and connect to the gauge as per instructions. Be particularly careful if you have a nylon vacuum pipe. It is very easy to put a kink in it! Replace the A-Pillar trim, removing any cable and pipe slack. Now screw the pod into the holes you prepared earlier and cover with the screw covers. I connected the illumination cable to the dash dimmer dial. If you have an S14a, it doesn't have one, so you'll have to connect elsewhere.
Job done, and one well smart gauge pod!