In my truck the standard sub was blown and making fart noises, I disconnected it but the speakers sound very tinny. I replaced the blown sub with this sub from eBay, £22 delivered, it's 8 inch instead of the 6 inch original as I couldn't find a 2ohm 6inch at a cheap price.
Blown original.
Made spacer/mounting board as it won't fit in the standard plate.
Used the standard amp and connector, so it could be disconnected and removed easily.
This is the final mk2 version, I had to mount the board in front of the bracket as there isn't much depth in the panel. Had to cut away a lot of the bracket but it's easy to keep enough to give a good mounting. The trim panel fits over it ok, I could cut more depth in the trim panel but it's not worth it as it simply bows out slightly and isn't noticible.
The sound is pretty good, it's not the same as having a proper sub and amp in a ported box, but for general use it's fine. You can feel the bass through the frame of the truck so it's doing it's job.
For £22 and no loss of space in my truck it's a big improvement!