My wife's FXT is a lease, as it is with all her past cars - so I've had to improvise a bit, to get a "fully removable," without-a-trace, install.
I route the antenna wire via the V-shaped silver trim panel. It is rather easily pried open, with the instructions from the NASIOC thread below:
The 1st comprehensive 2008 impreza dash & radio removal guide - NASIOC
You can then take it under that black side panel, and simply string it out and into the door-jamb. Don't worry, the weatherseal there is so soft and pliable that it'll just crush around your antenna wire, with no problems to either it or the wire. Trail enough slack so that the wire follows the contours of the door-jamb area; you can either tape it in-place with some "Gorilla Tape" or the like, or just leave it as-is. It's not going to hurt anything, nor have anything hurt it, for that matter.
^ That's from my laser-jammer install, and as you can see, the wire is much thicker. Same idea, here.
Now take the wire and pass it through that gap in the bodywork, where the door, fender, and A-pillars meet. It's still quite warm outside, so don't be afraid to pull on that fender with your hands - just get it open enough to allow the wire to pass through, and you're all set.
Now, slide that trim-cap between the hood, windshield, and A-pillar over:
In this one, I'm routing the wire "down" and into the engine compartment. That's not what you want to do. You want to go "up," the other way, into the wind-seal that goes around the perimeter of the windshield glass.
Use a plastic trim-tool or even an old and un-used credit-card: carefully pry up the outer wind-seal around the windshield, and lay the wire underneath. Work your way up to the roofline.
Note that the pictures I have above routes via the driver's door. My XM antenna is routed via the passenger side.