I've never taken apart an STi headlight unit, but based on the oem GD/GG impreza's headlight, one way would be to:
(1) unplug all connectors
(2) remove the eniter headlight assembly from the car
(3) remove the bulb where condensation occurs
(4) air the headlight unit in a dry, warm, ventilated area
(5) re install
Actually, I'm surprised there is condensation at all to begin with. Perhaps one of the bulbs are not securely fastened, allowing moisture to be trapped inside?