Maybe that helps: there is always one (or rather never more than one) strongly declined element before the noun.

Ich fahre das blaue Auto. (Definite articles are always strong)

Ich fahre ein blaues Auto. (Indefinite articles are most often weak so the adjective is strong)

Eines schönen Tages. (I said most often. Genitive singular indefinite articles are strong, obviously)

Thinking in terms of strong and weak declination is key. But maybe it's obvious. It wasn't for me when I learned about it in linguistic lectures in university.