Stock radio's have no RCA jacks on the back, if that's what you're asking. You can still run a sub if you want. Just make sure the sub amplifier can accept Hi-level/speaker inputs (which you can tap into the rear speakers) or you run what is called a line converter. Line out converters tap into speaker wire, but convert it to RCA jacks. Just make sure you go with a quality brand as these devices are notorious for picking up signal noise. Not so bad for subs, but definitely bad if you ever amp your door speakers. A new deck is the best solution for this obviously, but a line out converter is a good <20 buck fix if you love your 6 disc.
I made a box for a 12" sub in my old truck. Check my old cardomain site here:
I can't give you any specs, as I built it on the fly. Just check your measurements, find a sub with the least amount of top mounting depth you can, and go from there. Just picture the subwoofer basically cradled by the box. The bottom and back of the box will be real close to the basket itself. It not the optimal box obviously, but you can make it work.
Get a grill for the subwoofer, and then you will have the maximum amount of room possible because the seat can then rest against the grill and not interfer with the sub movement.