I bought a 08522-B1010 replacement horn; why are there two different bolts?

One of the new bolts(and spacer?) will be a match to what's already on your car. Use the matching one for the new location(passenger side) and reuse the existing one for the original side(drivers).

 

I installed the same kit a couple years ago. I just know I was able to do it with just the parts in the kit(I had some leftovers).

 

I just installed this Premium dual horn upgrade kit today. It’s a great upgrade! The kit came with everything needed including some spacers and longer bolts which are not all needed for the Prius but may be required for other models.

I made a short video showing how I did the install:

https://youtube.com/shorts/XwnIUXTjHBI?si=q_r9l0heK63YYrZm

 

You didn't install it quite correctly, but the difference doesn't matter. Instead of repurposing one of the hood latch bolts, you're technically supposed to use an open hole that's symmetrical to the existing horn location. It's one of the reasons for the extra bolts in the kit. But your way works well enough.

If I think about it tomorrow, maybe I'll take some pics or a short video to show my install from a couple years ago using the same kit. I've been wanting to do a little touch up of the wiring that I skipped back then anyway. Just some zip ties to clean it up a bit.

 

The instructions that came with the kit did show the horn mounted to the hood latch and it does fit there.

 

Yeah, but those instructions aren't for a Prius(assuming they're the original instructions in Japanese). Did you notice that none of the diagrams matched your car and the other horn in the drawings is nowhere near where it is on a Prius?

Again, it doesn't really matter because the hood latch location works fine, but there's a second mounting point on the passenger side of the radiator core support that's specifically for a second horn. The Vietnamese version of the Prius comes standard with a second hockey puck style horn and that's where it goes. The only reason I point it out is because I get a little obsessive about keeping things as OEM as possible on my own car.