Learn French: En effet, ces utilitaires ont l'air rouillés.

With the expression avoir l'air (to look / to seem), the adjective agrees with the subject of the sentence when it describes the subject's physical state. Here, it is the vans (ces utilitaires) that are rusty. Since utilitaires is masculine plural, rouillés takes an -s.

Are there cases where the adjective with avoir l'air agrees with l'air instead of the subject?

Yes! If the adjective describes the 'expression' or 'manner' of a person rather than the person themselves, it can agree with the masculine singular noun l'air. For example, Elle a l'air idiot (She has a stupid look). But for inanimate objects like vans, the adjective always agrees with the object itself.

I thought utilitaire was an adjective. Why is it used as a noun here?

In everyday French, un utilitaire (short for un véhicule utilitaire) is commonly used as a noun to mean a van, light truck, or commercial vehicle. It is masculine, which is why it takes the masculine demonstrative ces and the masculine plural adjective rouillés.

Why use En effet instead of En fait here?