What should I look for to get my radio to work when the GPS functions fine?

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I ahve a GPS/radio combo with the cassette behind and an Alpine disc changer in the trunk. My GPS functions with no problem but my radio will not come on. This happened before and I unplugged the changer, pulled the fuses to check them, powered off the dvd navigation player. And eventually the radio worked again. I believe that it was a fluke that started working. Where should I look next?