[SVXdc]

Yes it's possible. The factory XM antenna cord is the one that plugs into the socket at the very bottom of the HU (far right side, when viewing the rear). Blue socket in this picture. That plug looks very similar to the one from the car's GPS antenna -- look closely and compare the upper-left corners of the two sockets in that picture.

In other Subarus (WRX, STi, Forester, etc.), that plug is at the end of a short "feeder cord", which connects from another plug down at the floor below the HU.

Subaru has different feeder cords for those cars when they have the non-nav OEM HU. The alternate feeder cords have a FAKRA connector (rectangular plastic plug, usually pink colored) at the HU end. It's very easy to remove the pink FAKRA shroud, leaving a standard round SMB connector (which will fit an aftermarket Sirius/XM tuner box).

I haven't been able to look in a BRZ to see whether it has a similar factory plug. Can you check for that? The feeder cords are typically around 12 to 20 inches long, so you should be able to see the lower end at the bottom of the HU cavity after you remove the HU. But in some cases, you must remove a small trim panel on the left side of the transmission hump (running above the carpet, near the accelerator pedal) to reach that junction.

If the cord that is plugged into the rear of your OEM HU is long and doesn't have that junction anywhere nearby, there is also an aftermarket cable intended for Lexus models which will work. The downsides with that one are it's 12 feet long, runs about $70 shipped, and you'll have to shave off one of the keys on your car's plastic plug. Search for SFA12F. Subaru's cords are around $20, and an appropriate one won't require you to modify your car's plug.

Incidentally, the square sat antenna and GPS plugs are made by Sumitomo, from their HFC series (High Frequency Connector). The GPS antenna plug is part number F(6098-2724) "Atype". Sat radio plug is F(6098-2961) "Btype". But you won't see those numbers mentioned anywhere by vendors selling cables.

The SFA12F cable fits the Atype connector (GPS plug), thus the need to shave the key. On other Subarus, the lower connector is usually the Atype, but could be either gender -- varies with car model.

If you find that you have the connector at the floor, I can post some Subaru P/Ns on feeder cords you can use.

Bonus points: While you're looking, follow your car's AM/FM cord down there. You may find it's a short feeder cord like the XM cord, with a different connector at the floor. On the other Subarus with the OEM Fujitsu Ten nav HU, the lower connector is a standard Subaru style AM/FM plug.