In BRZ models, the factory GPS antenna is on a small metal bracket fastened just above the HU. In all other Subaru models, they place the GPS antenna in a "pocket" above the instrument cluster, just below the dash. BRZ has a similar spot, but for some reason Subaru didn't use that.
In all recent Subaru models where Sirius/XM was installed by the factory, the sat radio antenna is on the roof, usually a combo AM/FM + sat unit.
In models (besides BRZ) where sat radio is added by a dealer or the owner, the OEM kit includes a sat radio antenna that you stick inside the windshield, at the far upper right corner.
As I mentioned to sroby via e-mail, BRZ has versions of the various antenna "feeder cords" that include separate coax cables for AM/FM and sat radio. See these diagrams:
2015 BRZ AUDIO PARTS (ANTENNA)
2015 BRZ AUDIO PARTS (RADIO) SYSTEM
There are alternate feeder cords for cars without sat radio (I guess BRZ models outside North America [ADDED: and some Canadian "Sport" models that did not get a sat radio-capable HU]). Those same P/Ns probably match up to Scion parts for FR-S models, and Toyota parts for GT86 models.
If you want to swap in the BRZ combo rooftop antenna, you'll need to either:
A) Swap in the corresponding combo feeder cords. I haven't worked out which parts you would need.
B) Keep using your existing AM/FM feeder cord, and add a DIY cord for the sat radio portion. I haven't seen the bottom of the combo antenna, so I don't know what kind of connector you would need at that end of your new cord.
Added: The farther north you are, the more you'll really want the sat radio antenna to be on the roof.
Added2: Good pictures of the antenna's short "pigtail" cable and its plug are several posts down in this thread:
DIY - 2015 OEM Sharkfin antenna retrofit