Clarion radios must be removed to gain access to the label on the back, this is simple and requires removal keys as they free the head unit from the mount.
Once your radio is removed there will be a label on the back, this is where your serial number & part number will be located.
For further help in removing your Nissan Clarion radio, please see this YouTube video
To find your Nissan serial number, you will need to remove the radio using radio release keys. Once the radio has been removed you will need to locate the radios label, which is on the top or side of the radios casing.