Can I Swap 2006 Radio into 2005?

Almost all car stereos have auxiliary inputs on the back of them, even if the car manufacturer doesn't wire that into the interior so you should be fine. I can't think of any other difference, but we'd need to have a photo of the back of both systems to be sure...

There's a JBL upgrade for Gen2 Prius but pretty sure that's just an amp under the seat and some slightly better speakers, but no changes for that upgrade would be related to a different head unit.

 

Back when I did car stereo installs wiring diagrams were printed on all units, but that was many decades ago. In cases when there was not yet a car stereo installed we'd use 9v batteries to test the speaker wires connection to speakers and a test light to find 12v power controlled by turning ignition key and 12v power that was constant with no key in the car.

Only other thing worth mentioning that's unique to Prius is that it's integrated into the touch screen, so if you put in an aftermarket stereo you either have to install a resistor or leave old stereo still connected under the seat to prevent from getting error codes logged.

 

In your old generation 2 you're going to have all the required plugs for the 2006 radio to just plug right up and work seriously that's what should happen on some of the radios there's a plug up there that doesn't get used and it's just sitting there and if not all three of them are plugged into the back of the radio because there's places for them to go That's that before you button up if you want turn the mess on Make sure it all works but that's about it in a nutshell You shouldn't have to rewire anything unless you're adding things or making big changes