There are two type of clarion CD changers CBus and CNet
Cnet is the later type. The Fiat standard items are CBus
Therefore if you buy a Newer Clarion Head unit (which will be CNet compatible) you will have to buy a Cbus to Cnet converter. BUT the 12 disk unit does not work with this

So if you want to keep using your 12 disk chager you must find a Clarion HU that is Cbus. I am not ceertain if Clarion ever produced a CBus compatible CD HU that plays WMA.
Therefore I would just ignore the 12 disk changer, sell it on , and get a Head unit of your choice that can play WMAs.