I've just put 6x9 3 ways in the rear of my ute. Just modded the stock metal plate that the 5x7s were mounted on. Fine atm. One day when I could be bothered I'll make some nice wooden brackets to mount the speakers to.
With the space in there, you could put a couple of 6x9s. (3 or 4 even?)
Or maybe some 10" subs. Not sure about the depth though... Maybe not quite deep enough for a sub...
I've also replaced the 5x7s in the doors with 16cm, 3 ways.
Had an amp installed. A 10" sub in a box lives behind the passenger seat. All Pioneer ofcourse.
Sounds very, very nice.
Eleventy billion times better than the stock 5x7s.