Your Siemens VDO CDR 500 radio comes with an antitheft feature. It will display "Locked" on the screen when your car dies or when someone removes the radio on your car. To unlock your car radio, you need to have an unlock code. You can obtain this code from your user manual, or you can obtain it from your Siemens manufacturer. With the code in hand, you can automatically unlock your radio in just a matter of minutes.
Consult your user manual. It should have an antitheft card that displays your radio's serial number and unlock code.
Contact your Siemens manufacturer and ask for an unlock code, if your manual does not have one. Provide your radio's serial number. Write down the unlock code.
Enter your car and turn it on. Wait for the radio to turn on. Press the "Tune" button and enter the first number of the unlock code, using the preset buttons.
Repeat the previous step to enter the rest of the code. Your radio will automatically unlock.