All passenger cars are required to carry at least two rearview mirrors; which one is mandatory in every case?

Correct answer: The left side-view mirror.

The left exterior mirror is required in every case. The interior mirror is no longer required when it does not give you a good view behind you, for example because the load blocks the rear window, and the right mirror then becomes required. That is why neither the interior mirror nor the right mirror is always required.

Real Decreto 2822/1998, de 23 de diciembre, por el que se aprueba el Reglamento General de Vehículos