I tried this setup before, The bass is not deep enough, but the car vibrate like hell.. it is better to install in woofer enclosure. It will sound better if you have 2 woofers and build an Isobaric (push & pull) box for them.

Isobaric won't that up much space, the size of the box only about 1cubic ft. This is how isobaric look like.. you can take it in and out anytime you want, by using banana jack.