I just got a '99 s-10 with a factory AM-FM

I just got a '99 s-10 with a factory AM-FM Cassette all in 1 unit. It also has an aftermarket 6 CD changer. The disc plays but I can't get sound because I can't get the 'CASSETTE/AUX' button on the radio to switch off the radio. The button doesn't do anything,that is except when I turn off the radio then press the button. Then the little cassette icon on the radio will flash,but still no sound from the CD that's playing.