Plenty of room for 5 cubic feet in a single cab. Just takes 3 steps,
1. Remove bench seat and install buckets (if buckets come factory, ignore this step)
2. Toss passenger side bucket seat. (dumpster, out the door, highway, where ever you like)
3. And finally, install said box in place of passenger seat and your good to go. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Seriously though, go with what you feel with work with the room you have. Im running 1 18" at the moment and love it, and I prob could cram a second one in my trunk, but im sure it would not sound as good even with 2 because the box would be a little too small and suffocating the trunk.
If you think it will fit, go for it. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif