Hi Robin.
I've just been looking through my Vectra Radio Manual to look into a similar problem. It lists the Blaupunkt CAR300 in here and the method of entering the security code is as follows
1. Switch on ignition
2. Switch off the radio button
3. Hold the AS button and switch on the radio. The display shows CODE and after 3 seconds shows _ _ _ _
4. Enter the code in using the radio preset buttons for each character of the code. For example - typing in 0123 would mean pressing preset button 1 once, preset button 2 twice, etc..
5. Store the code with the AS button. Press the button until a beep is heard.
Adam Goldsmith, June 2005