Trailer wiring problem on 2001 Outback

[DS]

The left turn signal on my boat trailer is not working. I checked all the
obvious things and found although there was power to the right side and to
the running lights, there is none to the left. I have traced back the wiring
to a little black box under the carpeting on the right hand side of the
storage area. I didn't even know there was such a thing and I suspect it is
the problem.
What is this thing...it has wires from the car wiring compartment on one
side and the wires to the trailer harness on the other.
Is it something high tech and expensive or routine and easily replaced.
If I'm wrong, what else could be the problem? I've checked all the obvious
stuff like bad bulbs, corroded fittings, broken wires, fuses etc. etc.
Any help/advice much appreciated.
ds

 

[Edward Hayes]

I would start by using a test light (any auto parts for 5 dollars) and
see if your getting power to the "black box" I suspect that the box
gets power from the left side lights. Find the wire where it connects
to the light and you'll probably find it not connected. There is a pop
off panel on each side that allows you to get to the back of the bulb
sockets and wiring. Connect your test light clip to a good ground and
go testing for power.

 

[Bonehenge]

What is this thing...it has wires from the car wiring compartment on one
side and the wires to the trailer harness on the other.

That's an isolation box, it electrically isolates the trailer from the
rest of the car and converts the two color tail/turn system of the car
to the trailer's single color. Some modern cars will display false
"bulb out" indications due to the trailer lights. The black box
prevents that.

As someone else pointed out, use a test bulb to see if the indications
going _in_ to the box are working. If not see your mechanic. If they
are, replacement boxes are available from any local trailer hitch
shop. The black box is usually not a Subaru part.