I have a 2008 Prius.. All has been great except for the radio lately. In the last few weeks the radio will all of a sudden go into an uncontrollable loud static blast. I have to turn off the power to the radio in order to stop it. It continues as long as the car is running. Once turned off the radio is fine until the blast happens again. I have replaced the radio on warranty, but it still has the problem. I also have an external XM Radio attached, but I don't think that's the problem... Any suggestions as to what's wrong?