I remember seeing a different shot of the Moritz Mustang where it suspiciously looks like the repainted CC/Gröpper car, so it's very likely that he bought the car for the 1988 season. Both cars also ran the original style grille while most others switched to the newer one, which later was applied to Moritz car aswell, as seen in the other picture of the car earlier in this thread.
This isn't the picture i was talking about, but managed to come across this while I was putting this post together. I think this is just more proof that this is Gröppers car.
Another dutch car that comes to mind was the Bolderheij Sierra that appeared at the 24h Spa in 1988. Should be the car on the outside in the picture just after the start. Other cars visible include a Alfa GTV6, 635csi and a 325i.