Does not know where it goes. Ok, find a different dealer because that is a bad first sign.
But I'll tell you where it goes. The Toyota aftermarket 6-disc is a 1DIN high unit, which fits conveniently in the compartment directly below the existing radio. That compartment with the door that flips down. I've seen it in a couple of Prii and it looks nice and is controllable with the steering wheel controls.
If they order it for you from their parts desk, full instructions for installation should come with it. If not, then I would insist they get instructions before tearing apart the dash to get it in there. And you should insist that they get the dash back together and without squeaks before you are satisfied with the install.
Had an aftermarket store install the TOYOTA Cd Changer. It fits in the dash cubby. Works great and with wheel buttons.
Tried to get Toyota store to do this when I bouight the car but they were "unable"