Buying a 2005 Prius tomorrow (maybe!)

Yeah, if it's the upper section that's the LEDs. I had a cracked tail light on mine when I bought it and I replaced it with a TYC one from RockAuto. It fit perfectly and if I ever have any trouble with the other side I'll do the same. For $86 it's a lot easier than cracking open the light and replacing individual LEDs.

I would also take a peek at it before you order any parts though to make sure a connector didn't come loose or something. There are different connectors for different sections of the tail lights so if just the one came off it might cause this too. It's probably not that, but when you don't know who's been in there before you it's best not to make assumptions.