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I used to deliver used ex-contract cars to auction sites for a living. I liked to have the radio on when I was driving but, on hundreds of occasions, the radio needed a code entered. So I became an expert on entering the codes!
Assuming that the standard technique (used by Ford and others) is needed and, for example, assuming a code of '3784', you need to do the following:
Press button 1 repeatedly, until the first digit shows '3'. (If you accidentally press it too many times don't worry. Just keep on pressing it; you'll get there eventually).
Press button 2 until you've got '7' as the second digit.
Press button 3 until you see '8' in the third position.
Press button 4 until you see '4' displayed.
Then press button 5 to complete the code entry process.