The Ronda was derived from the Ritmo/Strada but according to Wikipedia was "redesigned just enough so that Fiat could not sue SEAT on the basis of patent infringement after 1980". So while it's still heavily based on the Fiat, it's changed enough to warrant a "Model origin: Spain" I'd say, same with the Marbella which is a redeveloped Panda. However, all the Seats with the same names as Fiats should have Italy as the model origin country.

How about having two flags for cars that have more than one country of origin? Such as the aforementioned redeveloped Fiat based Seats having both an, Ford Contour which was a collaborative effort between Ford of Germany and USA, Lada/VAZ 2101/2107/Riva which are all highly modified Russian/Soviet versions of the Fiat 124?