I got the Infinity Basslink in and installed over the weekend. To my surprise it's actually two 10" subs on each side of the unit.
The install was by far the easiest I've done on any vehicle. On the sport tracs you can pop off the drive side floor/kick plates as well as the running board door plates (just compression clips). There are existing wiring trays right underneath the plates, so no need to find a spot to squish it through and hope it doesn't rub away the rubber coating. I used a snake and starting from the rear and ran it up the trays, once out the front, just tape on your wire and gently pull back through. I ran the power on the driver side, and the RCA and Remote wire on passenger side. The ground can be used anywhere underneath the seat, the body/frame is right there. I used the torque bolt for the seatbelt clip, just make sure to scrape up any existing paint to give you a good ground contact.
As far as the space of the basslink I would recommend against it. It is just a bit too big for under the seat. I had to unbolt the seat to slide it in, and then bolt it back down, it's not an exact fit and does push the seat up. There isn't any issue with leg room for the rear passenger side rider, but the seat is not very comfortable and feels like your up in the ceiling.
It's in there, and is great, I'm surprised with the bass output. But if this thing ever goes dead, I think I'll be opting for a custom-made box with an amp under the rear driver side.
http://www.supercrewsound.com/commerce/product.jsp?prodId=28&catId=72
100 for the box, maybe 200ish for a sub, and probably 200ish for an amp to power it...I mean it's 300 more than I paid, but if it fits correctly, I would go that route.