Answer: Relative clause is "that are not registered." It is essential to the meaning of the sentence and, therefore, does not require commas.
Step-by-step explanation:
A clause that starts with a relative pronoun (who, that, which, whose, where, when) is called a relative clause. The purpose of these clauses is to provide more information about the noun that precedes them. In this case, relative clause "that are not registered" is used to describe the noun "cars." It is essential to the meaning of the sentence, since it tells us which cars in particular will be towed to the nearest impound lot. If a clause provides important information, it is considered essential and does not require commas.