Wait...you fear someone trying to carjack you.

While nobody can say the risk is zero, I can say it's laughably remote. Especially in that area there are tons of French plates. Keep in mind the Spanish side of the Pyrenees tend to have better ski resorts since they are the best mountains in Spain while demand in France is concentrated on the Alps.

Northern Spain also tends to be the wealthiest part of Spain and so there is much less property crime there, too. Regardless, foreign number plates are very common (there's been a car with Swedish plates parked on my street in Madrid for awhile now) and nobody actually cares.

Violent crime in Spain is exceedingly rare and the biggest problem is theft in areas with a more depressed economy.