Reverse polarity

Haven't seen as many posts about this for gen 1, maybe just because there weren't many gen 1s made. These are common posts in gen 2 and gen 3 forums.

Often, one of the casualties will be the DC/DC main fuse in the underhood fuse box, so checking that wouldn't be a bad place to start. (Of the multiple gen 1 underhood fuseboxes, this is the one on the left inner fender, 'left' meaning, as it always does in a car, on your left when you face the same way the car's pointed.)

It's a 100-amp fuse in gen 1, and I believe it has legs with bolt holes; it doesn't just pull out of slots. It's mounted in its own little modular island in the top inboard corner of that fusebox, and I think you unclip and lift up that island and you can get to the bolts holding the fuse in. (Don't do this with the battery connected.)

Sometimes other fuses join in the fun, so there might be more to check, but that's where I'd check first.

 

If you do a search for "reverse polarity" you'll come up with a whole bunch of hits, some of them relevant to your '02: https://priuschat.com/search/974367477/?q=reverse+polarity&o=relevance&c%5Bnode%5D=87

If it is the 100 amp fuse, there are directions for getting to it (not an easy task, apparently). Since you do have signs of life (horn) you may not have blown the fuse.