Looks like JL AUDIO is going to make a sub that may fit in the spare tire.
Here are some specs.
12TW5-4
Nominal Diameter: 12 in. (300 mm)
Continuous Power Handling: 300 W
One-Way Linear Excursion (Xmax): 0.38 in. (9.7 mm)
Voice Coil Diameter: 5.50 in. (140 mm)
Nominal Impedance: Single 4 ohm
Mounting Depth: 2.50 in. (63.5 mm)
Recommended Sealed Enclosure: 0.63 cu.ft. (17.7 liters)


Here is the link. JL AUDIO