Depends on the speaker.
Most speakers for car audio are designed to be used infinite baffle...i.e. no enclosure, which is why most people do not enclose the speaker itself, they instead install the speaker solidly in the door and seal the door.
There are those few car audio speakers, and home audio speakers (which some people use in their car), which would benefit from an enclosure. But without knowing the parameters of the speaker, there is no way to tell you for sure. But the vast majority of car audio speakers are intended for infinite baffle, so chances are very good that an enclosure would actually hinder performance.