aftermarket cd-radio

wondering if anybody can give some advice. My factory Delco CD-Radio has failed. I drive a yukon xl, 2001. The original delco was an RDS radio-cd. I purchased a kenwood cd-radio and an GMOS1 interface. The
kenwood powers up; but no sound comes from the speakers. The only thing that works is the onstar. I read in the fine print that the GMOS1 interface allows replacement of the factory radio (nonRDS) and maintain the onstar. Any thoughts, suggestions would be appreciated. Thanks.
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My sister's Grand Am had that problem when we installed an aftermarket stereo. It turned out to be the aftermarket stereo wasn't telling the car's stock amp to turn on, hence no sound.
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