Installed a used radio from a wrecking yard into our Chevy HHR, now the display says 'locked', how do we unlock it?

The newer ('03 and newer for example) factory radios are a different animal.

There is no actual "security code" in the radio as in past model years. The newer theft deterrent system utilizes the class 2 serial data to determine if the radio is in the appropriate vehicle...in other words it compares the VIN it has stored in it's memory with the VIN of the other modules (body control and PCM) in the vehicle to "check" to see that it's in the correct vehicle.

Each time the radio receives the "run" power mode message, it compares the VIN information it has stored to the VIN information received from a module on the class 2 serial data circuit responsible for transmitting that information. If a mis-match occurs, the radio display will indicate to the user the radio is locked. Once this takes place the radio will not respond to any buttons/commands and becomes inoperative. But the dealer can reestablish the radio into a different vehicle with a scanner.

Thank you for using fixya and your going to have to take it to the dealer and have them reprogram the radio to match the HHR vin number.