You can always get an OEM harness. It's plug and play. The aftermarket ones, while cheaper, seem to involve splicing. A hitch (Draw-Tite or Curt) from etrailer is cheap and easy to install. I installed a Draw-Tite on my 2016. I did not install wiring, though, as I just use it for a bike rack.
I would recommend you to install a hitch from any official services center, they take the full responsibility of installing we run a towing company and have a lot of towing trucks which we use for Blocked Driveway Removal in Queens, etc and we always install a hitch from the service center.
What year is your Forester? I think most of the newer Subarus come with the end of the plug hidden behind the back seat on the driver's side. eTrailer has the other end for cheap, just plug it and rout it. (Not sure what years constitute "newer" but my '11 OB had that)
2015. Yes, they come with the plug on the driver's side rear panel, behind the back seat. The install video is pretty good too. Should be a fairly painfree install.
Where are you located? I just put a hitch on my 17FXT I'd be happy to give you a hand if you wanted to come to Farmington.