My 2013 Prius Two wouldn't start one day. The 12V battery was fine, but I needed to provide a jump at the front through the jump clip in the fuse box. A jump put on the battery in the trunk wouldn't work. It turns out there was damage to the 12V cable that goes from the battery in the trunk to the fuse box under the hood. The power wasn't making it forward. The cable is aluminum (instead of copper) and something damaged it. My local shop ran a new cable to replace the damaged cable, and everything worked. If you're having a similar problem, check the continuity from the battery to the fuse box.
It's aluminum. If there is a scratch through the insulation, the aluminum will corrode. I think that's what happened.
I asked the shop what they saw. Something had clearly whacked the cable under the car. The plastic panel under the car was also banged up. Probably hit a rock or something on the road. I'm not the original owner, so it could have happened before I got it.
