Originally posted by Trixter Put the seat where you like it and then measure the area behind it. Measure from the seat to the back of the cab on the bottom and top and also the height.
Let's just say for example you have 10" on the bottom, 5" on the top and 14" tall (measuring straight up the back of the cab, not the seat.) Using these numbers now you need to konw how wide to make it so you get 1.5 (if you want seperate boxes) or 3 (if you want a single box) ((If you build a single you may have trouble around the drive shaft hump.)) If you used 24" wide you would come up with 1.46cu'
All I did to get the number was make a box that was 5"x14"x24=1680. Then another one only cut in half 'cause it's a triangle 5"x14"x24/2=840. 1680+840=2520/1728=1.46. (The 5 and the 10 worked out well for the example.)
Once you come up with the numbers you have to work with, it's just a number game from there to get the vol. you need. Make sure the box is deep enough for the depth of the sub.
Hope this helped...at least a little.