Yeah there is heaps of room under the dash, but there is nothing there to mount them to. You would need to fab up some sort of box for them to fit into.

For speakers to work corretly, they need to be enclosed. There is a fomula that you can use to calculate the exact volume required, but unless you are aiming for a comp car, then its not neccassery, but if you dont at least box em, they will sound horrible. No depth of sound, just tinny crap. Worse than the standad speaker setups that you got in cars of the 80s.

The other thing to think about is the direction of the speaker. The speakers would have to face down, and this would not be the best position for a mid range or tweeter. For a sub i doesent matter significantly its direction, but mids and tweeteers really benefit for correct directional placement.

It all comes down to what you want from the stereo. Lots of noise, or clarity at low or high volumes, strong bass response, clear vocals, and it all depends on budget.

The eaisest option would be to mount some 6x9s in the rear parcel shelf if its a sedan, or in the rear quarters if its a wagon, but I believe its a ute?

You could box some speakers and mount them overhead between the seats, or behind if you can spare the room.

The utes ar a challange to fit stereos, but its doable.