Just get a generic trailer adapter and wire it into the tail light harness. It's not really too complicated. The one I got was kinda crappy and it uses power from the light harness to run the trailer lights which causes the BRAKE light on the dash to come on when you hit the brakes. Other than that, it works fine.
On a subaru I used to have, I made a trailer light controller with 4 or 5 relays. Essentially made an AND gate and an XOR gate, so there's that option, too.
Now that I think of it, the wagon may have a place to plug in a trailer wiring harness. My sedan didn't, which is why I had to tap into the wiring harness.
You've tapped into the wrong brake light wire. Those models that had the brake light fail warning lamp have 2 brake light signal wires (on RHD models, this is the right side lamp, I'm not sure if this is reversed for the LHD models).
There is the wire from the brake pedal to the lamp, and then one wire out of that lamp to the other side lamp. You need to get the one from pedal to the lamp, not the one going between the 2 lamps. This then taps in before the "sensor" thingy, not after it.
BTW, this extra wire
On the sedans, the wire just goes across the back of the car. for the wagons, it goes forward to the back seat & then runs across the back floor and then to the back.
Edited by wagonist, 23 February 2014 - 11:49 AM.