He had them connect his Sirius portable module (that's never had a problem) to the Pioneer's aux in on the back of the deck.
No matter what the volume on the Sirius unit, as you increase the h/u's volume there's a very audible high-pitched whine. It happens when the car's engine is both on & off. It's not alt whine at all. It only does it when playing something through the aux input.
Any ideas what could be causing this?
He took it back to the shop that installed it and they told him "it happens" and that he can't do anything about it. I call BS... sounds like an excuse to me, but I've been wrong before.