Radio unlock codes don't work

I looked at the other thread that gave the sequence and codes to enter to unlock the radio but none of them seem to work. I enter the code and hold auto-m + 6 but nothing happens. It just eventually goes back to Err.



I read on another site that if the radio has been locked from entering an invalid code 3 times, the radio cannot be unlocked and you have to buy a new radio.



Is this right or am I doing something stupid wrong?