Oops. Didn't catch that part about the music comes out of the right speaker when only _either_ the right or left Rio Car AUX RCA jack is connected. I had to read it about three times just now to get it into my head.

All I can tell you is that I have mine hooked up to provide a three-conductor mini-phone jack as input to the empeg's aux and it works fine, so it's not a design fault or anything.

Maybe your sled's wiring loom is broken? It might be possible that there's some loose wiring in there touching something that it shouldn't. Hooking up an RCA output directly to the Aux inputs, one at a time, avoiding your adapter, would at least point the finger in the right direction. Of course, you may not have anything available to do that with (I don't think I would).