Did the seller not have any advice on the installation? The Aquamist diagrahm is a generic accurate layout of the components, exact location of those components is usually provided by the dealer who is selling the end unit.
I can tell you that the red wire from the fia2 goes to an ignition constant on 12+V source ? the Yellow/Blue wire at pin D2 at the ECU works well.
The green wire from the fia2 goes to one of the injector switched grounds at the ECU ? the Pink wire at pin D1 at the ECU works well.
WRX ECU pinouts
The pump should be located
near the battery somewhere within reach of the included
pump harness - where it will get adequate airflow so the heat sink case disipates heat rather than accumulating it.
The HSV is heat sensitive and should not be placed over the engine along a fender rail works well.
The jet can be located anywhere before the throttle body.
The pressure switch should be also be located away from the heat of the engine along a frame rail or the bulkhead works well. I know you are going to use the UTEC switch but it is a good idea to include the mechanical switch as a backup in case the transistor in the UTEC goes out in a closed state and runs the system continuously - set the mechanical switch lower than the UTEC switch and make the circuit such that both the UTEC and the mechanical switch are triggered before the fia2 begins firing the HSV.
The fia2 can be anywhere you wish in the passenger compartment ground at a good location near your choice.
Just some ideas.