Specmo will often be able to figure out your part number from your year/make/model (especially on a car like a CTS-V)
but if they claim they can't you can call up your local dealer's parts dept. and have them look in their parts catalog using your VIN
or you can ask your dealer's service advisers if they will scan your radio with a TECH2 to get the part number
(some dealers will want to charge you for this even though it only takes a couple minutes)