That is, indeed, the correct code for the given serial number.

Where did you get the serial number from? If it was out of the handbook then I would suggest that you pull the radio out of the dash in order to read the serial number from the label that's on the radio iteself. Car handbooks can and do get swapped around. So do car radios.

The "Wait 30 minutes" suggests that you have attempted to enter the wrong code too many times, or the correct code incorrectly. It might have permanently locked the radio, but suggest you try what I've suggested first.

Make sure you enter the code correctly.

You are using the correct procedure,but the code you have is incorrect for the radio fitted.Press buttons 2 and 6 together,this may display the M and 6 digit serial number.If this does not work,then the radio will need to be removed from the dash.Not too easy on these 3 tier systems.(it is actually a 1 piece radio made to look like 3 bits).