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If the antenna is plugged in, the unit may be pulling the ground from the shield. I've had that happen plenty of times.
As for your noise...I don't have any experience with these units yet. But, from what you are saying it sounds like you might have tapped into a noisy 12volt. Try to temporarily apply 12v directly from the battery to your constant and acc wires and see what it does. If it gets rid of your noise, try to find another 12v source...try the ignition harness. There are also noise filters for the power source, similar to what's found on the power lines of pioneer cd changers. I think radio shack may carry something like this to clean up noisy 12v sources.
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