1995-1999 Actually I believe (but have not physically verified) you can also connect some radios from 2000-2004 Outbacks - the main issue is the socket on the back of the unit that the wire harness plug plugs into.
I just bought my 99 Outback and currently have a factory P114 AM/FM/WeatherBand Cassette-only installed. I'm looking to replace it with a factory AM/FM/Cassette/CD unit (P117? 95-99 generation or P121? 00-04 generation).
I've searched www.car-part.com for local salvaged units. For 1995-99 Outbacks they list 8 different factory radio models:
AM-FM-cassette, Sdn, GT
CD changer, Limited model
AM-FM-cassette, SW, LHD, Outback
AM-FM-cassette, Sdn, SUS
AM-FM-cassette, SW, LHD, Brighton
AM-FM-cassette, SW, RHD
CD changer, exc. Limited model
AM-FM-cassette, SW, LHD, GT
AM-FM-cassette, SW, LHD, L model
AM-FM-cassette, Sdn, L model
I am also keeping an eye on eBay auctions - I have been able to physically compare models and their socket/plugs as most auctions have pictures, which has been educational. Most units here have been tested and come with a warranty.
Also if you want to know how to rip apart, er, I mean disassemble the center console and dash to be able to remove your radio you can find instructions by Googling it
or e-mail me (rbrow at pacbell dot net)