Okay, with the car off, and the heat/AC fan OFF do you get any noise? It sounds to me like you either have a dirty power source (ideally, all would come from the same circuit and be grounded together), or a cable somewhere with bad shielding. It's normal to pick up RF interference on a car with an antiquated pointed ignition system (like an Integra ). In most cases a ground loop isolator or noise filter can be added to the power supply to reduce it. The best ground is still the battery. Where you have it works fine for most cars.