Thanks for the link...that's a very informative site.
But on checking further into the existing wiring it now looks like the previous owner did not wire up the yellow radio mute lead from the CARK-91 to the CDR 500 connector so that explains why I do not have radio muting during phone reception. However I notice that there is no connector on the end of the yellow lead. Anybody know where I might get one to fit into the plug which mates with the socket on the back of the radio? Regards Patrick.