Try the following procedure to look up the Honda radio head unit's serial number:
1. Turn the ignition switch to ON.
2. Turn ON the device and confirm that the display reads CODE. If you don’t see this message, pull the backup fuse for 1 minute, and then go back to step 1.
3. Turn OFF the device.
4. Press and hold the 1 and 6 radio preset buttons, and turn ON the device.
5. The display toggles between two screens: U with the first 4 digits of the serial number (U2200 for example,)
6. And L with the last 4 digits of the serial number (L0055 for example).
7. Ignore the U and L and record all 8 digits.
Then go to this
Honda Radio Code Retrieval site and follow the instructions to get the radio unlock code.