Answer:
"Because we always wanted to go to the zoo."
Explanation:
This sentence is only one clause and needs another -- it's best combined with "Therefore, we piled in the car for a trip to the zoo". They can be rewritten as "Because we always wanted to go to the zoo, we piled in the car for a trip there.", or "We piled in the car for a trip to the zoo because we always wanted to go there."
An alternative is to say "We always wanted to go to the zoo; therefore, we piled in the car for a trip there." A semicolon is used here to separate the two clauses. It can also be written as "We always wanted to go to the zoo, and therefore we piled in the car for a trip there." The first sentence is more 'correct' though.
"there" is used because the word "zoo" has already been used before. Of course, using "the zoo" twice is grammatically correct -- it's just that using the word "there" makes the sentence more natural and less repetitive.
