Compuser wrote:

Success.

Thanks all for the info.  I didn't look at the wires in the bundled
harness itself, only at the wires leading to individual bulbs (I
finished before I saw the post from S).  All the bulbs but the ones for
the brake light had two wires each, and one was always black.  After
fretting a long while, posting here, digging though already packed boxes
for my old multimeter, I decided on a simple solution for the three wire
brake/tail bulb: I guessed.  Even if I chose incorrectly the first time
(I did) it would take about 90 seconds to pop the tail light again, cut
the lead wire going to my control unit, and slap a new 3M wire connector
on the other wire.
Wish I thought of that before wasting too much time - I could have been
packing more stuff or seeing my girlfriend again.

So, at least in my particular WRX, the green wire is the one going to
the brake filament, and the red one powers the tail lights.

Got my hitch and wiring module from eTrailer.com which I heard about
from a post on this newsgroup a month or more ago.  I forgot who wrote
it, but I'm glad you did.  Parts as follows, which cost about $175
including shipping - way way cheaper than Subaru parts and a little
cheaper than getting it installed at U-Haul, who says they don't have
compatible wiring and I would need to get that done by Subaru:

1     Ball 1-7/8x3/4x1-9/16 2000#
1     3593 Class I Drawbar
1     24710 Class I Hitch
1     Modulite Wiring Kit - from battery to tail
1     Modulite Lite Module - box to prevent stressing stock tail wiring

It was a pain to route the wire under my car leading from the battery in a way that avoided all hot, sharp and moving parts.
The hitch itself was pretty easy to install - I used only hand tools and the car remained on the ground because I don't have ramps or a lift.  As part of the install, you have to remove both rear tiedown/tow loops, and the outside exhaust hanger.  My draw-tite hitch has a method for securing the bar attached to the muffler, but it didn't fit well at all.  I tried at first to drive leaving that hanger unattached, but it bumped and rattled against the hitch during some situations like shutting the car off and getting off the throttle quickly.  I was able to get the rubber isolation exhaust hanger re-attached well enough that it stops the vibration.  You can't see it from behind the car, but it isn't ideal.  I bet the solution they designed for is for the Outback Sport and 2.5TS Impreza vehicles which have a single exhaust tip instead of my (stock) dual setup.

But everything works, and I'm moving from Massachusetts to Arizona starting Monday.  I hope the 4x8 enclosed trailer rides better with some weight in it, because driving it empty to my apartment didn't give me a warm fuzzy feeling.  No side to side swaying I could see, but it bounced and induced some drivetrain vibration that sort of felt like I had a bad tank of gas.  That quieted down when I drove over the only quarter mile of smooth pavement in the Boston area.

Once I get internet access setup at my destination I'll post again and
say how the hitch and trailer performed.

- Byron