zapwizard

It does help if everything is grounded to the same point, but it is not required.
What is more important is that the ground of your audio signal (or shield) is NOT connected to the power supply ground.

You need to double check your wiring for the amplifier and the PC.
A ground loop isolator won't help if the audio ground is still connected elsewhere.
A good amp will completely isolate the audio ground from the power ground.

Another possibility is that your alternator is directly affecting your amplifier when the voltage changes. In this case a power filter is what you need. They are dirt cheap at any auto parts store. I used one before my amp and LCD (had noise) along with a ground loop isolator.