enclosure for speakers?

They might sound better in an enclosure.  And that is a big might.  Component woofers for cars generally like a bit of space, so door cavities fit the bill pretty well.  If you damp all the metal skin inside the door and seal up the panel real well with caulk and tape, you can get a pretty good seal.  The problem with enclosures in a car is the size you have to make them;  generally smaller than the woofer would like if you want to get them to fit and still have some legroom.  The result can usually be found in a diminishment of the low end response, and you would hear this deficiency by the lack of punch in the 80 to 100 Hz region.

It's the bass response, not the midrange, that utilizes the large airspace.  Components like 6.5's have the capability of bass response and will blend with the sub bass response.  Midranges need very little air volume.  It just happens that the 6.5 will produce both bass and midrange response.