Hi Frank,
My car is also a Corsa with a Grundig Car 200 and I had to reset my own recently following a battery disconnection.
Make sure that the radio and the ignition are both turned off.
On the radio, press and hold down the 'AS' button, then turn on the ignition.
After about 3 seconds the radio will beep. Release the AS button.
The display should show 'CODE 10'. This refers to the number off attempts you are allowed to input the correct code.
You enter the code using the buttons 1,2,3, & 4.
For example, if the code is 5678 press,
button 1: 5 times
button 2: 6 times
button 3: 7 times
button 4: 8 times
When you have entered the code, press and hold down the AS button again until it beeps and your radio should now be reset.
PS
If your code has a zero in it, treat it as a 10.
Hope this helps,
Brian
Brian, May 2005