Unlike the trailer hitch installation instructions, the wiring installation instructions left a bit to be desired. Basically, you can’t get the trim panels off the interior until you know where they hid the snaps and latches, but you can’t find the snaps and latches before you remove the trim panels.
N.B.: this applies to a 2015 Forester. Subaru deliberately moves the connector around for each model and year, for reasons that certainly make sense to them.
Remove everything from the back end of the car that isn’t firmly affixed.
Remove the rivets from the left-side foam block by prying their heads with a big screwdriver; maybe you can get a needlenose pliers under there. They’re surprisingly difficult to get out, due to that nasty barb on the end:
Release the left side of the trim strip across the back of the compartment by pulling the front corner forward to unlock the latch that engages the trim panel on the left edge. Then you can pull the strip upward to locate the green rivet about a foot from the left end, stick a screwdriver under there, and pry it out of the frame. I didn’t do that, with the unhappy result that the rivet stayed in the frame:
If that happens to you, just pry the rivet out of the frame, slide it into the trim strip, and (when the time comes), ram it back into the frame.
A captive plastic cover over the bolt securing the cargo tiedowns yields to a fingernail, if you know that’s what you must do:
Remove the bolt and tiedown, which greatly simplifies pulling the side trim panel away from the frame:
The trailer hitch connector lies tucked far up inside the recess, taped to the wiring harness with blue tape. Slit the tape, pull the connector free, worm the aftermarket wires up in there, latch the connectors together, and reinstall everything in reverse order.
Subaru could, if they wanted to, add a foot of wire to that harness, tape it near the bottom edge of the trim panel, and eliminate half an hour of dealer labor charge. I think I understand why they don’t do that, but I don’t have to like it.
It’s faintly possible that someone with very thin arms could reach the connector without dismantling the butt end of the car, but the fingers on the end of that arm might not have enough strength to latch the connectors.
The electrical box fit neatly against the rear of the compartment, behind the foam block. A bit of razor knife artistry carved a notch for the fuseholder and the wiring coils up neatly inside an existing recess:
Until I install lighted hitch nuts (you could look it up), I think deploying the trailer connector through the hatch makes more sense than running the wiring through one of the holes in the spare tire well and exposing the connector to the elements. I don’t intend to do much trailer hauling …