Wow, all those answers and no one knows what the problem is and how to fix it. It is an easy fix.
You first need to find out if it is coming from your speakers or sub. If you have no sub then you know where it is coming from then. First thing is to replace your rca cables, especially if you used the cheap ones, the ones have a lack of insulation. If that does not fix it, go to an audio shop and pick up a ground loop isolator, this is not a fix but a band aid, sometimes this works to stop the whine from being heard, the isolator cancels it out. If nothing has worked at this point then it is your amp that is bad. I have the same problem, replaced rca cables did not fix, put in the isolators did not fix the problem, now I have to replace the amp. That is all you can do, the lower you put your gain the less you will hear the whine from your alternator. This also has nothing what so ever to do with the power wire at all, that is even absurd to even think it, the other thing is that your ground is not even the culprit. The only way you have a bad ground is if you did not sand off the paint, or if the ground connection is loose, those are the only bad grounds. All of these people that have tried to help have no clue to what there talking about what so ever.
Actually Andy, you need to tell the truth, the isolator may or may not stop the problem and the isolator is only a band aid for the problem.
