Nav trouble

Hey everyone!
I need some navigatrion help. I am sorry in advance that I do not have a lot of details but I am trying to trouble shoot my sisters 2004 Prius back in New York from my current location down here in Florida. Thankfully my 2008 is fine.

My sister says that her navigation has been going out. She brought the car to the dealership and they said her entire nav system needs to be replaced. They can give her a referbished unit for $800 + labor. The symptoms are as follows:

The screen will turn black but she can still hear the voices. The screen will come back on its own. She also gets an error message "external system not connected". She says that this could happen at any time, not just when the navigation system is being used. Also when this does happen it is as if the whole system crashes because she cannot control the AC\heat and the radio goes out.

In my research i would say this was a nav ECU problem but I want to double check and so I am posting. I would like to know if the ECU from other model years can be used (assuming you carry over the same DVD version).

It seems like an easy enough repair, go under the driver seat- remove the 4 bolts- unplug the ecu harness connectors- reverse steps with new ECU

I would be most grateful for any help! Thanks