I spent a long time trying to enter using source button until I found the following and it works.
ENTERING RADIO CODE
To enter the radio code you will need to locate the 'THUMBWHEEL' and the button below this.
Entering the radio code is straight forward, simply switch on your radio, using the 'STALK CONTROL' rotate the thumbwheel until the first digit of your radio code is shown on the display of your radio, then press the bottom button.
Next you will rotate the thumbwheel again until the second digit of your code is displayed, then you will press the bottom button.
Repeat this sequence until you can see the 4 digit code displayed on your radio, once you press the bottom button for the last time hold it until the radio beeps the radio will activate.
MARTIN JONES
February 2011