Fire up the car. You should have no whine. If you do, you can be guaranteed that a component is bad

Actually, if he gets a ground loop in the "Bench Test" mode, it doesn't necessarily mean a bad component. You can still induce a ground loop that way. For example, if a higher resistance wire (say, a wire that is too thin or has a loose crimp) connects the amp ground than connects the empeg ground, or vice versa, then you might get a ground loop.