I had the same thing happen after disconnecting the battery - the radio wouldn't activate without a security code. Unfortunately, the previous owner didn't provide the laminated card that has the code on it, and it wasn't written in the owners manual (nor should it be).

So, after giving the VIN to the dealer, they obtained the proper code from the distributor that day and had another card made soon after. If your car is grey market or not properly registered with the distributor, you may have trouble finding out the code.

Dean Albrecht
94 E500