In the 2003 Impreza at least the purge solenoid is in a finger shreddingly dumb location under the passenger side intake manifold. I just killed half an hour looking for it on a nice modern AVLS-equipped 2006 Impreza 2.5i before just tracing the hoses.
The FSM has a not very helpful diagram, and google image search has no records of this purge solenoid location. As a public service, I am posting it here so that future generations may bust their knuckles where I have gone before.
Here it is, behind the alternator, in clear view.
It appears to be the same part as before.
Top hose goes to the intake manifold, bottom hose goes to the supply pipe coming from the fuel tank.
