Bro I've an Aqua and I can only guide you about its battery fan vent. Its beneath the back seat behind the passenger seat. If you look carefully you'll see a vent. It will be close with clips. You just need to bring out the clips carefully to reach the fan. Once you remove the clips of that side only and bring out the cover you'll see the fan. In Prius i think the fan is reachable from trunk, but not sure about Prius.
I've got mine cleaned, it was full of dust and almost chocked. After getting it cleaned the efficiency of battery and milage have been improved.