OK! Here's my suggestion. Possible gound and/or possible power.
Alpine recommends on all of the 60x4 headunits to run the constant wire directly to the battery. These headunits pull so much current, that the stock wires for constant do not handle enough current. I would recommend using at least a 12awg wire and perferably a 10awg wire.
Our radios have to be grounded seperate of the factory harness. I would check to see that you indeed have a good ground.
One other suggestion, do you have the infinity system in the car? This factory option has an amplifier, which could be introducing the noise. If so, I would recommend bypassing the amp and running wires directly to each speaker. Yes, I know the wires are hard to get through the door boot, but it will sound better once it's done.
Here's another thought that I just had. You have components. Where are the crossovers mounted. If they are in an area in which they can come in contact with power wires, this too can introduce alternator whine.
