"Where is my parents' car?"
Translation:Kde je auto mých rodičů?
The plural possessive is "rodičů". It is not declined together with the noun (it is a "non-agreeing attribute") and therefore comes after it "auto rodičů".
However this position rule is very often disregarded with modern English-like adjectives. I find it to be an insult to the language but that is the only thing I can do about that.
In singular you can choose between the agreeing and non-agreeing possesive: "tátovo auto"/"auto táty", gen. "tátova auta"/"auta táty".