Terry's reply was excellent. Unlike him I do use an isolator between the car battery and the battery (Optima) in the trunk. I didn't want to take a chance of draining the car battery. However, the isolator is a major source of RFI - it can almost wipe out lower portions of the AM broadcast band. Ferrites on the power cables help but are not a cure. Just too lazy to fix it!! 73 steve WE5X