Bummer... I always go into stuff like this hoping that there is just something simple and easy to do to correct the issue. You should be able to find a used OEM stereo at a local salvage yard for around a hundred bucks, maybe they'll let you test it in the parking lot? At least that way you'd know whether the problem is in the radio or the wiring in the car. Unfortunately, you've already done everything the dealer service department will do to diagnose the issue. They replace radios for this kind of thing.