Short answer: for a wiring adaptor that will plug into the SVX's stock radio harness, see this link:
http://www.subaru-svx.net/forum/showthread.php?t=6530
(that's the same link that soobiesvx93 posted above). There is also a link to that from my SVX Audio Installation Guide, which I gave a link to above. When I said that had "everything you need to know" I wasn't kidding
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Not sure if you're aware of this, but in case not: Every different head unit manufacturer uses a different plug on the radio side. (Nearly) every different car manufacturer uses a different harness on the car side. You'll need two pieces to wire your new Rockford:
- A Rockford harness (which should have come with that radio; replacements are obtainable if, say, you bought it used and the seller didn't include it), and
- An SVX "wiring adaptor" (harness that connects to the car's factory harness). I'm the only person who makes these. See the above link to get one.
You will then need to solder or crimp those two harnesses together.
Read my guide. It's all in there!