On my 2006 XT it was easier to locate in the rear passenger door sill loom. The signal originates up front but has to make it back to the reverse lights somehow... I can't speak specifically to your model year. A sure fire place to find it is at the tail lamp itself, but then you have to double your wiring back to the front.

 

Same as shown in this thread. Your factory wire color is still BROWN/YELLOW.

I think the reverse signal is in a similar location in most Subarus (all models). There is a harness on the left side too -- the wires go back to the respective taillight assemblies.