Do you have a radio like this in your VW passat? 
Is it in “Safe” mode after the battery has been disconnected?
If you don’t have the owner’s manual (like me), you should be able to find the code in your wheel well located in the trunk under the mat. It should look like this:
See in the picture where the 4 numbers are circled? This is my code
Locate your code which should be on a sticker like this one in the wheel well. Turn your radio on which will say Safe for a short time and then say 1000. Since my code is 0700 I pressed the 1 button twice to make it change to 0.
Then I go to the next number, then push it until it says 7.
When I finished I pressed the FF button (held it down) and voila! My radio was back on.