Jumped prius opposite polarity and now the car is completely dead. What should I do?

Pretty sure that means you have to replace your fuesable link... Not sure what else... Maybe @ChapmanF know?

 

Go through its pockets for loose change?

(other ideas....)

If you see the door and check engine lights, it's only mostly dead.

 

Have you visually inspected the fusible link yet, as described in the linked "other ideas"?

 

Pretty dang sad that a dealer can't sort this out.
The initial troubleshooting can be pretty simple and may just be tracing the 12v.
Verify the battery terminals have 12v.
The fuse box should have a stud/nut connection where the (+) cable from the 12v battery fastens. Make sure you have 12v there. If not, something between the stud and battery terminal is open. Often there is a high amp fuse or link in that circuit, sometimes mounted on the + post of the battery.
Often, (but I'm not a Gen 3 person) there is also some type of high amperage fusible link between that stud and the rest of the fusebox.

You just need to determine where the 12v is lost, assuming the fuses did their job and protected from any major electronics getting smoked.

Yup....the 2015 Prius (+) battery terminal cable has a 140 amp fusible link/fuse in the vertical section next to the battery, covered by the red plastic. May want to check that. If it's bad, it may require replacing the entire piece. You would think the car would be completely dead if it were open. With the dash still having a couple lights, it would make you think the problem lies somewhere after the battery cable/fusebox stud, but you just never know.

 

Post #3 has a link with details on checking / replacing that.

 

Taking apart two cars' underhood fuseboxes to move the 125 amp internal fuse from one to the other sounds like maybe the single most difficult possible way to test that ....

 

That's the one I was thinking of. Is it the one you swapped?

 

If you look back at the link I posted a week ago, that thread has a lot of information on how to get access to that. It's also something I would try to just test with a multimeter first; there are some ways you can test that without taking the box all apart, and that's a lot less work.

 

If the 125 amp fuse is blown. replace it. There is a thread linked in post #3 that shows how.

After that, perhaps other fuses may still need to be checked.

 

I believe the right way to replace the 125a link is to remove the whole box which has big “christmas tree” connectors on its right side going into the sheet metal. I have never blown or bypassed that 125a link but that style bypass was used by another member and worked until his bad connections failed.

As Chapman says there are always other fuses blown as well. In that box and in the interior fuse box. If a bypass was tried with a wrong sized fuse it will blow as well. There are other fusible links in the engine fuse box that some fail to check. There is a fuse at the 12v battery.

Other common mistakes: Checking fuses by pulling them out and not returning the fuse to the right space.

People will pull the hybrid battery orange disconnect and fail to perform the final slide engagement.

It is possible the 12v battery is too discharged to start.

It always requires two Ready attempts after disconnecting the 12v battery, which is sometimes needed to clear codes.