Question about Pioneer FH-X840DAB incl. DAB antenna

don't you normally just need a vehicle-specific "adapter" - bezel to square/DIN like on my Seat Ibiza Cupra?

You must measure this exactly. This radio has the following dimensions: width 18cm, height 10cm (DIN2) and installation depth 15cm. It is quardatic, so it will not fit neatly in most modern cars with special asymmetrical panels. Then craftsmanship is required.