I have a 2002 that the navigation screen is completely white and the radio will not work properly as if the navigation unit is taking over the sound (only rear speaker come on and column cannot be adjusted) - this happens only when key is turned and navigation screen comes on. If I didn’t have the key turned and only have the radio on, everything is fine. I tried taking out the fuse that controls Nav unit, audio works fine then. I thought the navigation screen is the culprit so I bought a replacement but it is doing the same thing (front speakers muted and volumb cannot he adjusted). What do you think is causing this?