shoot, its been awhile since I've done a WRX, but if I recall, there is a nice fat fat grommet above the clutch high in the underdash. This will fit even 0/1 sized cable nicely.
to prevent having to take apart the car, you can just remove the door sills, and any panel directly on the carpet. (I think the lower B-pillar will ahve to come out.) You will be able to lay the cable somewhere under the sill or in that area with ease.
doesnt your car have a series of wire travelling from front ot back there? in a series of clips? if that is the case, you can just clip your power wire inside those clips, and the carpet will fold back with no bump or bulge.
then simply repeat on the other side for your signal.