I would check that you have a K Bus connection between the changer and
the Symphony, you also need to make sure that the above K Bus connection
is connected to the car's K Bus circuit.
The 75 relies on the K Bus circuit to enable the Symphony to "Talk" to the
changer.
The K Bus is connected to the Symphony via pin 19 on the Blue mini ISO
plug.
The K Bus circuit can be found on various connections in the 75, the closest
to the radio is on the back of the IPK.
It is possible you just have a loose White/Red/Yellow wire and this needs to
be attached to the wire on pin 19 blue mini ISO.
Dave T