this might be a completely noob question, but WTF does your audio componets and where they are grounded have to do with a blown heater core?
did i miss something? please tell me if i did. maybe this is just one of my stupid moments.
anyway, your battery is grounded to the chassis, and your amp to the chassis. you could try running a ground straight to the battery, just to see if it helps, im just not sure of the connectiong between the heater core being blown and the audio system.