wait.. there no screeching when you swtiched head units? there was no screeching in your car, AND the other car, or the other car screeched still no matter what?

if it is magnetic interferance, take the speakers out, run some new like 4 foot section of the stock connections in the door and hold the speakers like 4 feet away from metal.. . Ive never had anyhting like that ever happen before...