Radio Code?

Hello all.
I'm looking for the unlock code for a 'Grundig SC 303 (C) IT' radio/cassette out of a Vauxhall Cavalier. The radio has been out of use & in my loft for 10 years & I have a job for it now.
From the label on the side :-
Type SC 303 D (C) .
Serial number GM1303N3138828.
I could also do with a breif instruction of how to tap it in, I understand you use repeat presses of keys 1 to 4 to input the code then the AS key to confirm??