It still sounds as if you have no K Bus connection to the K Bus circuit in the car.
Pin 7 on the BMW plug is the K Bus connection.
You have established that the K Bus [White/Red/Yellow] is connected between
the Symphony and the CD changer, now you need to make sure that the K Bus
wire between the radio and CD changer is connected to the cars K Bus circuit.
If you look at the back of the IPK [speedo] you will see a White/Red/Yellow
wire in one of the plugs.
Connect your Radio/CD changer K Bus wire to the K Bus at the back of the
IPK.
You may be able to find a White/Red?Yellow elsewhere in the 75, any of
these will work ok.
Dave T