Clarion won't be compatible with Blaupunkt, so you'd need to remove the headunit to check what model it is. For example, the sticker on it may say 'Alfa 937 High', which is a Blaupunkt model.
If it's Blau, then it's easiest to source a factory Blaupunkt CD changer, as suggested. Apparently, the suitable models include CDC A03, A08, A09. Mind you, I've never checked what model they called the one that was in my GT.
Removing the side storage bins in the SW is a little awkward (based upon having removed them in my GT). From memory, there are two main screws inside the hole, then there are other screws. To access these, you need to remove the section above the suspension tower (which is screwed and clipped into position), and you may need to release the C-pillar trim. Once removed you can also peel back the carpet side boot trim and check if the cable is there.
I suspect it's not. You can of course check for the other end, behind the headunit. Remove the headunit (which you need to anyway to check the brand/model), and see what is plugged into the blue mini-ISO socket (interlinked with the green/yellow plugs), if it's Blaupunkt. There should either be a blank plug, or a plug with all wires for a CDC.
If it is a Blaupunkt, and you can get hold of a factory CDC, it should be plug-and-play. Mounting it into the boot/storage bin may require you to fashion some brackets (if not supplied) and drill some holes to screw it into place. You could of course mount it elsewhere.