There is no part number for the whole door . . . it's made of many parts. Each of the door 'shells' has a different part number (L/R). If you're buying a replacement from GM, you'll have to buy the parts that you need, have it painted, and assemble it. You'd be better off going to a salvage yard and getting a whole door, if that's what you really need.
Go to the document library at CadillacFAQ.com and get the CTS Parts list (scanned page 131).