It looks like someone might have gotten this to work with a similar HU from forester.
http://forums.nasioc.com/forums/show....php?t=2323331
The person bought a harness from svxdc to hook up a cheap ebay camera to the HU.
My biggest concern would be that the Forester does have the camera option, so the HU might have different firmware that enables the camera.
The if the impreza has a different firmware than the forester, then it might not automatically display camera on reverse.