You should be able to retrieve the serial number by just turning on the unit and holding down the tp button for 5 sec's then you should see BECKER if you turn the knob you will see the serial number (8 digits) take this number and go online to:
http://www.radio-code.co.uk/instant_online_becker_radio_code_check.php
Payment is around 15.00 US last time I used it, you will get the number online and emailed to you.
Hope this helps.
-T