In traditional grammar, the example is analysed as a passive as follows:
1...This car.......is............made................ of ............steel"
...subject .......verb...past participle...preposition... noun
Noun Phrase.[...........verb...............]..[adverbial modifier]
A more advanced version would have
2...This car............ is........ made......... of ............steel"
...subject .........verb.....adjective... preposition... noun
Noun Phrase....verb......adjective....[adverbial modifier]
Noun Phrase....verb.....[........adjectival modifier...........]
The test for an adjective is often "adding 'very' and see if it makes sense. HOWEVER, this does not work with ungradeable adjectives, e.g. *"She was very pregnant." (wrong) and "made" is an ungradeable adjective.
OED made(adj): Produced or obtained by making as distinguished in some way from other modes of origin or acquisition; resulting from human activity.
1981 A. B. Facey Fortunate Life 44 The road was just a winding track—there were no made roads in those days.