Any ideas on where to start with diagnosing electrical noise?

My problem right now is a damn annoying whine when my engine is off. All I've done is removed my alpine CD changer and installed the Riocar using all the same wiring. The alpine had noise when the engine was on, but not when it was off.

I read through the FAQ on installation and didn't get any ideas.

I also tried regrounding the rio with no luck.