there's a grip of options out there nowadays. They've really come a long way as far as getting subs into a truck, especially an extended cab with fitting them under a seat. Pioneer makes a pretty good low profile sub that should fit under the back seat if you have one
Amazon.com: Pioneer TS-SWX251 10" Flat Subwoofer with Enclosure 800 Watts: Explore similar items
Amazon.com: Pioneer TS-SWX251 10" Flat Subwoofer with Enclosure 800 Watts: Explore similar items