The FM601US came out of a 2012 Impreza or Crosstrek. I don't think that model (or ones from a WRX/STI or BRZ) includes a sub out. There's a small chance that one from a Forester does (but Subaru's wiring diagrams don't show it).
Pictures and pin-outs of the rear harnesses of the STI version are here
(see posts 4 and 9 in that thread). The Impreza version looks the same except for the model numbers.