Pine Hollow Diagnostics does a Prius v Head Gasket

RJ, you helped before with a challenge to my Prius Gen 2. I have another.
When it rains, the small battery appears completely dead, no power to the main fuse box in front. If I attach jumper cables, it starts immediately and will run until stopped, as normal. However, starting again yields the same dead battery/no connection blank stare from the cockpit. Days after the rain, it will start as normal. I look at the battery and it is perfectly fine, reading 14.2 volts. connections are tight.
I seems to me that somewhere between the battery and the front fuse box (where the positive lug for charging is), the battery cable can be compromised by the up-spray from the street during a rain or hitting a puddle or some shorting mechanism which doesn't seem to affect the battery itself (as an enduring short would).
Is there a way to trace the battery to front fuse cable route in a Gen 2 Prius to detect a disconnect, short or some cause of this power interruption between perfectly good battery and the starting mechanism?

 

You're reading the voltage via an in-dash menu, with the car on? That doesn't tell you anything about the state of the battery. At the least check the voltage at the posts, with a digital meter. If it's in decent shape it'll read at least 12.5 volts.

Then do the same check at the jump point, in the fuse box under the hood, to ground. If they're the same or close your wiring is ok.

To really see where you're battery's at, hook up one of the electronic load testers; they're around $50~100 USD. Most automotive retailers these days have them, and will test for free (good for business). Dealerships have these testers as well, but will likely charge to test (because they can...).

I'm using a Solar BA5 for such testing, easy to use. Current iteration is BA9 I think.