2004 Prius Technical Manual 67-2 and 71-1 would give you a hint.
01-5 gives you hints on how to deal with different fastening solutions,
but to get to the radio, it's only pop-out and screws...
The "Instrument Panel Register Assy" ( that vertical panel having the air
vents ) is a simple pop-out, hold the upper part and force the lower part out.
You don't even need any tools for this.
Once these are out, you have access to the screws.