replaced battery radio needs a security code
You can contact any Mercedes Benz dealership which can get you the code for your radio. Check the owners manual as perhaps the previous owner wrote it down by the radio operation section. Good luck. Cost about $20 for me a few years ago.
Search Mercedes-Benz M-Class Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercedes-Benz M-Class Experts