There is no trunk in a truck, and door speakers are variable and largely ineffective at producing deep bass. Whether you want great sound for recreation or at a work site, building a sub box will yield returns both in terms of listening pleasure and a sense of accomplishment. Approach this project with tangible results in mind. The object here is not having a larger woofer or louder volume than the next guy, but deep bass.
Adjust back the seat behind which you will place the subwoofer box. From the floor, measure how tall you want your box. At the desired height, measure the distance between the seat back and the back wall of the cab. If the last measurement is not at least 9 inches, adjust the projected height of your subwoofer down until the top is at least 9 inches wide, to accommodate at least an 8-inch speaker and a grille frame.
For any given woofer, find the published closed box volume. This volume is expressed in cubic feet or a fraction thereof. Your box size must be calculated to this volume plus the thickness of the MDF. Multiply the woofer's stated closed box volume requirement by 1,728: that is one cubic foot, or 12 by 12 by 12 inches. The speaker's size plus 1 inch is one of your outside dimensions. The inside dimension subtracts twice the MDF thickness, or 2 times 3/4 inch = 1 1/2 inches. Multiply that dimension times the inside dimension of either the desired height or the width of the box. Divide the resulting figure into 1,728 to get the remaining inside dimension. Adjust either the height or the width until you get the right volume.
On the MDF sheet, mark the outside dimensions of all six box sides. Allow for the width of the saw blade and cut. On the top panel, mark a circle large enough to match the woofer's inside flange diameter and cut with a saber saw.
Assemble the box with wood glue and screws. Drill a 1/4-inch hole in a side panel for the speaker wire. Thread speaker wire through the hole and toward the speaker hole, leaving 1 foot of wire in excess on both ends.
Split the wire 3 inches at both ends. Strip 1/4 inch at each end. Crimp on terminals to fit the speaker tabs, or leave bare for spring terminals. Connect the speaker. Stuff the box with pillow stuffing and drop in the speaker. Apply putty to the back of the speaker flange and fasten the speaker in place. Silicone seal around the wire exit hole. Connect to the amp and test.
