Join Date: Jul 2018
Location: Frankfort KY
Car: 2018 Outback Limited 2.5i
Feedback Score: 0 reviews
First off, polarity for horns doesn't matter, so you can hook it up either way around.
Second, I checked an online parts fiche, and it appears that Subaru used the same low tone horn since basically forever, and the same high-tone horn from 2015-current. It also looks like the parts fiche indicates that the low-tone horn is on the driver's side.
I don't know about Subaru's and Outbacks specifically, but typically there are two different methods that I've seen horns use. The first is a single spade connector on the OEM horn that carries the positive voltage, and then it grounds through the vehicle body (this looks to be what the Grimspeed harness is set up for). The other is a two-pin connector, usually something like a metripack or something similar. If this is what's on the car, your best bet if you want to avoid cutting is to purchase a matching plug, and make your own harness.
If you can provide a picture of the connectors on the horn, I may be able to identify the connector and give you a course of action; I'm at work and don't have my car with me. From some online parts photos, it looks like they may be a single spade terminal connector, but sometimes they use a generic photo.
Finally, what PDF are you looking at?