OK, you have bought some beautiful woofers, build a great box
out of hand-cut burled oak, and had the whole thing dipped in
gold, and wrapped in rich Corinthian leather by Ricardo Montebalm
himself. Now it has to go into the car. This also can make a big
difference to sound quality and sound pressure level.
This is all going to depend on the type of car you drive, and the type of system you are building. If you have just finished building a dozen dual reflex band-pass enclosures and are going to slide them into the back of your Cube van, then you are likely going to make out just fine without my help. But, if you built a sealed enclosure with a pair of Focal Utopia 10-inch subs, and you want them to blend nicely with your MB Quart or Dynaudio front end, and you drive a nice big BMW, Mercedes, Lexus or Infiniti, then you will want to be aware of how the placement will affect your sound quality.
Certain cars have inherent problems with bass. I have had the luck to come across three of them. 1989 Mazda MX-6's, Ford's Thunderbird last generation before the return of the little two-seater, and some older generations of Honda Accords. These cars, due to their dimensions, seem to simply swallow bass from about the 40-70 Hz range. Right where all the fun is.
The solution to these problems seems to be in firing the woofer in different directions (towards the rear seats, straight up, or towards the rear of the car. But this doesn't work every time. The subsequent alternative is to effectively alter the length of the vehicle by firing the subs through the rear deck, or rear seat, and seal off the trunk from the rest of the car. Your low frequency output will be decreased as compared to someone who can use their trunk, but you will have bass, and it should sound good.
Never be afraid to experiment. The S.A.S. Bazooka tubes can be a great way to get bass into your car. You can make enormous differences in low frequency output by simply adjusting the placement of these units. They're worth playing with for quite a while before bolting down.