No function from changer

Hi I just bought a '90 BMW 318is with Pioneer MEH-P5000RII Head unit and Pioneer CDX-P650 6 disk changer BUT the multi changer is not working!! I have checked that power is getting to the unit and YES 11.75v is coming to the lead but no function at the changer and the head unit does not register that it has a multi changer source connected. I have opened up the unit but could not see any obvious fuses. Does anybody know where these would be located within the changer.(if they exist) Or has the unit seen it's last days. Could you suggest an alternative changer that would be compatible with the existing head unit and cabling? THanks Michael