First , check that you are using the correct code. Ask another question quoting the SERIAL NUMBER ( should be M******.
Check also that you are entering it correctly.Instructions here :http://www.davidsonelectronics.co.uk/code_entry.htm
If you are entering the correct code , in the correct sequence and it still doesn`t work, then it looks as though the radio has been recoded to a different number in the past by a Radio Decocoder and will have to be taken in to be read + recoded again.
It might be cheaper just to buy another off Ebay with a code.
RegWorts
May 2008