If the CD changer works without a problem and the problem is that there is no sound, I would suggest that there is a break or a loose connection from the HU to the speakers. Most likely that a connector which carries sound from the changer to the speakers is loose and therefore while the CD changer works, the sound it not getting through to the speakers.
Check the rear of the HU and ensure that everything is connected. Same with connections back in to the CD changer. Buying a Blizsafe or similar product that connects in place of the CD changer will not solve the problem.
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