The installation of a **sunshade on the passenger seat** is not mandatory, but it is advisable. This is because, although it is not specifically mentioned in the texts, sunshades can help improve **visibility** by reducing glare caused by the sun. However, there is no regulation that requires their installation in vehicles.

- In the case of **mirrors**, it is mentioned that the right-side mirror is recommended but not mandatory. This suggests that some elements that improve visibility, such as sunshades, may also be recommended but not mandatory.

- Additionally, the installation of **sun blinds** on the rear windows is allowed only if both exterior mirrors are installed. This reinforces the idea that elements that help control glare are optional, as long as they do not compromise safety.

Therefore, although it is not mandatory to install a sunshade on the passenger seat, it is an advisable measure to improve comfort and safety while driving.