According to Peter Morgan's Original Porsche 911, non-tail Carreras had it mounted at the top of the rear window in 1986, at the bottom in 1987, and then back to the top from then on. I'm not sure that it changed exactly by model year, as I've seen '87s with it at the top. The pod thing was for cars with tail spoilers. Here's inside and outside pics:
(boy are my windows filthy!)
Somehow it always seems to be in the way when looking out the rear view mirror.
Hope this helps,
Chris.