The 6x8 size is very uncommon and ford is one of the few auto. This ford expedition sub box is made to fit behind the third row seat. Web you could try to make the cutout a little bit bigger and mount an 8 sub there, but your problem lies in the fact that the factory amp is pretty small and won't drive most.
SoundBox F150 SuperCab (Extended Cab) 20092016 Single 10" Subwoofer
