Update: So I was able to find a used FRS antenna on ebay and gut it. The connectors are the same, it's just missing one of the pins on the harness (leading to the XM antenna). Leaving the XM off, FM works alright, obviously way worse reception with such a tiny antenna nub (I only made a simple wire antenna that was about 2 or 3 inches long). I'll revisit the antenna I soldered into the circuit board when I actually need to. Basically, I get all of the radio broadcast in my hometown -- in HD, but not very much from SF (~50 miles away and over a hill or two).
I plasti dip sprayed the G37 shell flat black since I didn't have time to color match to WRB. I'll take care of that at a later time. It looks a little dusty, and that's probably because I need to rinse it off.
tl;dr: it can be done to a USDM BRZ, it's just a little more complicated.
