The reverse gear signal would be an ORANGE/WHITE wire at pin #2 on the 28-pin harness. That was the case on 2013-2015 BRZ, and still is on 2016-2017 BRZ.
Either your car is really an 86, or those pictures are from someone else's FR-S or 86.
Pin-out for 2016-2017 BRZ is
here (courtesy of "sLoPy").