Many people wonder how to place the subwoofer box in their vehicle. To find the best placement in your vehicle, the best thing to do would be to take these recommendations and do a little fine tuning of your own to get it just right for your install. There are differences in recommended subwoofer box placement in cars, trucks, and SUV's.
In a car, it is usually best to have the subwoofer box as close to the front of the car as possible with the subwoofers firing towards the rear of the car. This generally will allow the subwoofer box to produce the best sound for your car. Firing the subwoofers upwards in the trunk is not recommended, as this will tend to make the trunk rattle more than normal. The exception to this rule for cars is if your car is a hatchback. If you have a hatchback, you can probably get away with firing the subwoofers up towards the glass and still get very good output from your system.
In a truck, if you can make it work, it's bets to keep the subwoofers towards the seats and fire them towards the rear of the cab. If this won't work with your truck, you can also get close to the same result with having the subwoofers fire upwards. You can also get decent sound out of a down-firing subwoofer box in a truck. If you're really short on space or placement options, you can get a setup that goes behind your seat and fires towards the seat, or a box that fits under a seat and fires up into the seat.
In an SUV, we have found the best placement for the subwoofer box is to be close to the rear door, with the subwoofers firing towards the rear of the SUV. Some people like the subwoofers firing upwards, but we have found this to not sound as good as having them fire towards the rear.
Even with taking these subwoofer box placement options into consideration, it will all come down to how your setup sounds in your specific vehicle. These are guidelines to follow, but it doesn't mean that a different placement of the subwoofer box won't sound better. Certain vehicles will make a sub box placement sound different than others. If possible, you should try a few different placements in your vehicle to see which one you like best.