Okay, then, time for the usual ground loop diagnostic.

Is your stereo an aftermarket stereo or the factory one? (I don't recall if you're still using an empeg?)

Either way, see if you can pop it out and wire the doohickey directly to the same wires that the stereo is powered by, with (just for experimentation's sake) the shortest wire run possible. Also see how short you can make the audio connection cable.

Also, just for ha-ha's: Um, is this the first device you've ever plugged into the Aux-in jack on the stereo? If it's the first, see if a different device (one that's not connected to the car's electrical system, such as a portable battery powered CD player, or the headphone jack on your cell phone) produces the same noise. If so, then the problem is still not a ground loop, but rather, a problem with the aux-in on the stereo.