Don't know where you can post - the closest thing I've been able to
find is a VW/Audi adapter at Crutchfield:
http://www.crutchfield.com/S-bGTPbSs5yYd/cgi-bin/ProdView.asp?s=0&c=2&g=113800&I
=12040VW16&o=p&a=0&cc=01&avf=N&search=Antenna+adapter

Is that what your picture looks like?

You can email it to me too, if you want...

Thanks for the help so far!  Bizarre that Subaru has no info on this.
Remember, this is on the end of the cable that plugs into the
*antenna*, not the stereo.

Hi everyone,

Just got the "VW/Audi" adapter in from Crutchfield, and it works!!  :)

The connector is threaded on the outside, but there's a sheath on the
Subaru antenna cable that fits over the threads, and the center coax
pin goes right in where it belongs.  Hooray!

Haven't heard from my wife on how the reception is, but I'm guessing
this closes the case.

BTW, thought I'd mention the kit from www.antennax.com was about 30
bucks, and the adapter piece from Crutchfield was about 7, with
shipping another 5 or 6(!), so total cost of the repair was about
$45...  plus my time.

Some advice on installing the replacement antenna (actually says
"Radio Shack" on it)-

The mast on the replacement antenna has 5 pieces on it - exterior: a
silver screw-on cap, a rounded chrome piece, and a gasket that is
intended to sit flush on the outside of the car; interior: a
half-circle piece and a smaller rounded piece.

I tried taking off the three exterior pieces and using the Subaru
factory screw-on cap, but the threads didn't match - when the antenna
came up, it literally came right out of the car and fell over!  So,
using the original thru-skin fitting didn't work. What I did was just
use the screw-on cap that came with it, and tossed out the rounded
chrome piece and gasket, as well as the interior pieces.

It sits nicely in the remaining Subaru gasket, and most people will be
none the wiser, so long as you attach the antenna securely inside the
car...

To install the antenna in the car, I had to take out the "base plate"
that the factory antenna was screwed to - I moved the electrical
grounding strap over to the new antenna, and used the mounting strap
provided with the new antenna to secure it to the car instead.  Don't
forget to put the drain line on and put it in the receiver!

If you're looking for a much easier install, buy the factory
replacement from Subaru - you can have it replaced in 5 minutes flat,
I'd bet, as the old antenna came out like it was greased. If I had an
identical replacement, back on the road in no time.

The AntennaX replacement was a kludge, but it worked with a little bit
of effort.  I'd chastise AntennaX for making it seem like it would be
a more precise fit...

Remember, this was done on a Subaru Legacy Wagon, so I have no idea
how it would work on other Subarus, but I'm guessing it's a similar
operation on Outback wagons, etc.

Good luck everyone!